Use executable finder to find xctool.
Use executable finder to find xctool.

Summary: This allows Buck to use xctool as installed by brew.

Test Plan:
CI
create an apple project
do not configure xctool in .buckconfig
install xctool from brew
run tests

LegNeato/buck,nguyentruongtho/buck,shs96c/buck,Dominator008/buck,janicduplessis/buck,LegNeato/buck,marcinkwiatkowski/buck,darkforestzero/buck,janicduplessis/buck,davido/buck,pwz3n0/buck,sdwilsh/buck,nguyentruongtho/buck,ilya-klyuchnikov/buck,ilya-klyuchnikov/buck,rowillia/buck,JoelMarcey/buck,raviagarwal7/buck,daedric/buck,SeleniumHQ/buck,sdwilsh/buck,bocon13/buck,mogers/buck,rowillia/buck,mikekap/buck,darkforestzero/buck,SeleniumHQ/buck,brettwooldridge/buck,brettwooldridge/buck,pwz3n0/buck,mogers/buck,artiya4u/buck,brettwooldridge/buck,OkBuilds/buck,Addepar/buck,OkBuilds/buck,illicitonion/buck,brettwooldridge/buck,rmaz/buck,Dominator008/buck,darkforestzero/buck,illicitonion/buck,mogers/buck,Distrotech/buck,sdwilsh/buck,Addepar/buck,darkforestzero/buck,Distrotech/buck,tgummerer/buck,rowillia/buck,shybovycha/buck,shs96c/buck,bocon13/buck,sdwilsh/buck,janicduplessis/buck,shs96c/buck,facebook/buck,ilya-klyuchnikov/buck,raviagarwal7/buck,rhencke/buck,k21/buck,davido/buck,Dominator008/buck,janicduplessis/buck,JoelMarcey/buck,artiya4u/buck,kageiit/buck,daedric/buck,daedric/buck,vine/buck,robbertvanginkel/buck,vine/buck,robbertvanginkel/buck,shybovycha/buck,raviagarwal7/buck,davido/buck,SeleniumHQ/buck,illicitonion/buck,shs96c/buck,mikekap/buck,mikekap/buck,LegNeato/buck,bocon13/buck,zpao/buck,illicitonion/buck,lukw00/buck,vine/buck,shybovycha/buck,justinmuller/buck,vine/buck,vine/buck,vine/buck,Addepar/buck,SeleniumHQ/buck,vschs007/buck,clonetwin26/buck,illicitonion/buck,mikekap/buck,shybovycha/buck,zhan-xiong/buck,vschs007/buck,k21/buck,zhan-xiong/buck,k21/buck,liuyang-li/buck,kageiit/buck,rhencke/buck,dushmis/buck,darkforestzero/buck,lukw00/buck,romanoid/buck,artiya4u/buck,shs96c/buck,liuyang-li/buck,marcinkwiatkowski/buck,daedric/buck,liuyang-li/buck,Learn-Android-app/buck,raviagarwal7/buck,OkBuilds/buck,vschs007/buck,grumpyjames/buck,rmaz/buck,mikekap/buck,dsyang/buck,grumpyjames/buck,rmaz/buck,davido/buck,lukw00/buck,JoelMarcey/buck,clonetwin26/buck,tgummerer/buck,dushmis/buck,zhan-xiong/buck,ilya-klyuchnikov/buck,zhan-xiong/buck,kageiit/buck,Dominator008/buck,OkBuilds/buck,rowillia/buck,Distrotech/buck,vine/buck,romanoid/buck,vschs007/buck,zhan-xiong/buck,shybovycha/buck,artiya4u/buck,zhan-xiong/buck,mogers/buck,LegNeato/buck,stuhood/buck,zpao/buck,stuhood/buck,liuyang-li/buck,zhan-xiong/buck,liuyang-li/buck,rhencke/buck,SeleniumHQ/buck,Distrotech/buck,rhencke/buck,robbertvanginkel/buck,dsyang/buck,daedric/buck,stuhood/buck,rmaz/buck,pwz3n0/buck,ilya-klyuchnikov/buck,dsyang/buck,darkforestzero/buck,grumpyjames/buck,SeleniumHQ/buck,janicduplessis/buck,lukw00/buck,sdwilsh/buck,SeleniumHQ/buck,davido/buck,rowillia/buck,vschs007/buck,mikekap/buck,davido/buck,grumpyjames/buck,dushmis/buck,ilya-klyuchnikov/buck,robbertvanginkel/buck,vine/buck,illicitonion/buck,grumpyjames/buck,marcinkwiatkowski/buck,OkBuilds/buck,LegNeato/buck,JoelMarcey/buck,clonetwin26/buck,illicitonion/buck,clonetwin26/buck,tgummerer/buck,rowillia/buck,romanoid/buck,OkBuilds/buck,sdwilsh/buck,rowillia/buck,dushmis/buck,robbertvanginkel/buck,artiya4u/buck,rhencke/buck,shybovycha/buck,vschs007/buck,tgummerer/buck,stuhood/buck,mogers/buck,raviagarwal7/buck,justinmuller/buck,shs96c/buck,lukw00/buck,SeleniumHQ/buck,tgummerer/buck,clonetwin26/buck,bocon13/buck,brettwooldridge/buck,dsyang/buck,robbertvanginkel/buck,dsyang/buck,stuhood/buck,mikekap/buck,daedric/buck,davido/buck,zpao/buck,grumpyjames/buck,k21/buck,LegNeato/buck,marcinkwiatkowski/buck,lukw00/buck,shs96c/buck,Addepar/buck,dsyang/buck,marcinkwiatkowski/buck,Addepar/buck,shs96c/buck,davido/buck,Dominator008/buck,clonetwin26/buck,vschs007/buck,romanoid/buck,tgummerer/buck,LegNeato/buck,zhan-xiong/buck,LegNeato/buck,facebook/buck,romanoid/buck,shybovycha/buck,zhan-xiong/buck,robbertvanginkel/buck,sdwilsh/buck,vschs007/buck,dsyang/buck,darkforestzero/buck,ilya-klyuchnikov/buck,sdwilsh/buck,ilya-klyuchnikov/buck,romanoid/buck,Dominator008/buck,Addepar/buck,Distrotech/buck,pwz3n0/buck,mikekap/buck,Learn-Android-app/buck,bocon13/buck,artiya4u/buck,dsyang/buck,JoelMarcey/buck,pwz3n0/buck,Learn-Android-app/buck,ilya-klyuchnikov/buck,zhan-xiong/buck,justinmuller/buck,dushmis/buck,davido/buck,vschs007/buck,rmaz/buck,Addepar/buck,tgummerer/buck,dushmis/buck,JoelMarcey/buck,marcinkwiatkowski/buck,Learn-Android-app/buck,Addepar/buck,LegNeato/buck,romanoid/buck,dsyang/buck,shybovycha/buck,vschs007/buck,rhencke/buck,bocon13/buck,dsyang/buck,nguyentruongtho/buck,facebook/buck,raviagarwal7/buck,justinmuller/buck,rowillia/buck,mikekap/buck,kageiit/buck,illicitonion/buck,facebook/buck,lukw00/buck,Distrotech/buck,OkBuilds/buck,robbertvanginkel/buck,SeleniumHQ/buck,rhencke/buck,marcinkwiatkowski/buck,daedric/buck,rhencke/buck,stuhood/buck,pwz3n0/buck,marcinkwiatkowski/buck,SeleniumHQ/buck,romanoid/buck,clonetwin26/buck,vine/buck,pwz3n0/buck,stuhood/buck,Dominator008/buck,robbertvanginkel/buck,rowillia/buck,sdwilsh/buck,romanoid/buck,justinmuller/buck,raviagarwal7/buck,rmaz/buck,bocon13/buck,nguyentruongtho/buck,brettwooldridge/buck,SeleniumHQ/buck,vschs007/buck,shybovycha/buck,rhencke/buck,facebook/buck,Dominator008/buck,davido/buck,vine/buck,Addepar/buck,Dominator008/buck,Distrotech/buck,facebook/buck,vschs007/buck,k21/buck,rmaz/buck,illicitonion/buck,Distrotech/buck,daedric/buck,justinmuller/buck,kageiit/buck,dsyang/buck,Learn-Android-app/buck,zpao/buck,k21/buck,Addepar/buck,liuyang-li/buck,shybovycha/buck,liuyang-li/buck,brettwooldridge/buck,kageiit/buck,SeleniumHQ/buck,brettwooldridge/buck,clonetwin26/buck,davido/buck,artiya4u/buck,Distrotech/buck,janicduplessis/buck,illicitonion/buck,romanoid/buck,raviagarwal7/buck,raviagarwal7/buck,janicduplessis/buck,mogers/buck,dushmis/buck,Addepar/buck,k21/buck,dsyang/buck,pwz3n0/buck,shybovycha/buck,mogers/buck,bocon13/buck,k21/buck,liuyang-li/buck,artiya4u/buck,lukw00/buck,zhan-xiong/buck,rmaz/buck,rowillia/buck,rmaz/buck,mikekap/buck,tgummerer/buck,justinmuller/buck,Addepar/buck,JoelMarcey/buck,Learn-Android-app/buck,janicduplessis/buck,mogers/buck,lukw00/buck,rmaz/buck,justinmuller/buck,ilya-klyuchnikov/buck,lukw00/buck,daedric/buck,LegNeato/buck,SeleniumHQ/buck,stuhood/buck,pwz3n0/buck,OkBuilds/buck,dushmis/buck,janicduplessis/buck,Dominator008/buck,zpao/buck,davido/buck,nguyentruongtho/buck,liuyang-li/buck,grumpyjames/buck,pwz3n0/buck,grumpyjames/buck,marcinkwiatkowski/buck,raviagarwal7/buck,janicduplessis/buck,liuyang-li/buck,mogers/buck,rmaz/buck,daedric/buck,LegNeato/buck,stuhood/buck,tgummerer/buck,dushmis/buck,JoelMarcey/buck,JoelMarcey/buck,janicduplessis/buck,marcinkwiatkowski/buck,ilya-klyuchnikov/buck,clonetwin26/buck,k21/buck,daedric/buck,kageiit/buck,zpao/buck,clonetwin26/buck,bocon13/buck,shs96c/buck,justinmuller/buck,shybovycha/buck,grumpyjames/buck,Learn-Android-app/buck,artiya4u/buck,grumpyjames/buck,robbertvanginkel/buck,dsyang/buck,janicduplessis/buck,JoelMarcey/buck,marcinkwiatkowski/buck,LegNeato/buck,shs96c/buck,darkforestzero/buck,OkBuilds/buck,Dominator008/buck,Addepar/buck,romanoid/buck,romanoid/buck,k21/buck,Learn-Android-app/buck,k21/buck,darkforestzero/buck,k21/buck,rowillia/buck,robbertvanginkel/buck,brettwooldridge/buck,mikekap/buck,shs96c/buck,shs96c/buck,justinmuller/buck,OkBuilds/buck,robbertvanginkel/buck,marcinkwiatkowski/buck,bocon13/buck,zhan-xiong/buck,dushmis/buck,shs96c/buck,justinmuller/buck,rmaz/buck,stuhood/buck,brettwooldridge/buck,raviagarwal7/buck,brettwooldridge/buck,clonetwin26/buck,liuyang-li/buck,Learn-Android-app/buck,illicitonion/buck,pwz3n0/buck,sdwilsh/buck,mogers/buck,marcinkwiatkowski/buck,tgummerer/buck,rhencke/buck,facebook/buck,Dominator008/buck,stuhood/buck,darkforestzero/buck,lukw00/buck,illicitonion/buck,ilya-klyuchnikov/buck,shybovycha/buck,rhencke/buck,artiya4u/buck,OkBuilds/buck,rowillia/buck,JoelMarcey/buck,ilya-klyuchnikov/buck,rmaz/buck,sdwilsh/buck,grumpyjames/buck,nguyentruongtho/buck,Learn-Android-app/buck,darkforestzero/buck,LegNeato/buck,robbertvanginkel/buck,sdwilsh/buck,raviagarwal7/buck,clonetwin26/buck,romanoid/buck,clonetwin26/buck,sdwilsh/buck,daedric/buck,bocon13/buck,mikekap/buck,JoelMarcey/buck,illicitonion/buck,JoelMarcey/buck,Distrotech/buck,brettwooldridge/buck,tgummerer/buck,grumpyjames/buck,vine/buck,raviagarwal7/buck,bocon13/buck,darkforestzero/buck,justinmuller/buck,daedric/buck,nguyentruongtho/buck,mogers/buck,Learn-Android-app/buck,zhan-xiong/buck,artiya4u/buck,OkBuilds/buck,justinmuller/buck,brettwooldridge/buck,k21/buck,OkBuilds/buck,davido/buck,vschs007/buck,Distrotech/buck,darkforestzero/buck,zpao/buck