appveyor: disable automatic builds by MsBuild
appveyor: disable automatic builds by MsBuild

Summary:
By default appveyor looks for VS solution/project file in a directory. Since we have custom build script, disabling this logic.
(see https://www.appveyor.com/docs/appveyor-yml/ for `build: off`).

Recently buck appveyor builds started failing (from time to time) with error:
Specify a project or solution file. The directory does not contain a project or solution file
(like in https://ci.appveyor.com/project/Facebook/buck/build/1.0.3854).

Test Plan: https://ci.appveyor.com/project/ilya-klyuchnikov/buck-qinnv/build/1

Reviewed By: coneko@fb.com

fbshipit-source-id: 94991ec

SeleniumHQ/buck,marcinkwiatkowski/buck,zpao/buck,k21/buck,rmaz/buck,Addepar/buck,marcinkwiatkowski/buck,brettwooldridge/buck,nguyentruongtho/buck,marcinkwiatkowski/buck,dsyang/buck,JoelMarcey/buck,kageiit/buck,facebook/buck,JoelMarcey/buck,zpao/buck,Addepar/buck,zhan-xiong/buck,Addepar/buck,ilya-klyuchnikov/buck,ilya-klyuchnikov/buck,romanoid/buck,shs96c/buck,romanoid/buck,k21/buck,JoelMarcey/buck,SeleniumHQ/buck,brettwooldridge/buck,SeleniumHQ/buck,nguyentruongtho/buck,robbertvanginkel/buck,shs96c/buck,ilya-klyuchnikov/buck,shybovycha/buck,facebook/buck,LegNeato/buck,JoelMarcey/buck,shs96c/buck,Addepar/buck,clonetwin26/buck,k21/buck,rmaz/buck,shybovycha/buck,robbertvanginkel/buck,ilya-klyuchnikov/buck,Addepar/buck,nguyentruongtho/buck,marcinkwiatkowski/buck,robbertvanginkel/buck,brettwooldridge/buck,robbertvanginkel/buck,k21/buck,clonetwin26/buck,JoelMarcey/buck,zpao/buck,marcinkwiatkowski/buck,romanoid/buck,dsyang/buck,LegNeato/buck,rmaz/buck,Addepar/buck,shs96c/buck,shybovycha/buck,LegNeato/buck,JoelMarcey/buck,k21/buck,romanoid/buck,zhan-xiong/buck,romanoid/buck,JoelMarcey/buck,dsyang/buck,brettwooldridge/buck,ilya-klyuchnikov/buck,kageiit/buck,SeleniumHQ/buck,shybovycha/buck,facebook/buck,k21/buck,shybovycha/buck,LegNeato/buck,clonetwin26/buck,shybovycha/buck,rmaz/buck,marcinkwiatkowski/buck,robbertvanginkel/buck,marcinkwiatkowski/buck,k21/buck,zpao/buck,ilya-klyuchnikov/buck,rmaz/buck,ilya-klyuchnikov/buck,zhan-xiong/buck,shybovycha/buck,clonetwin26/buck,ilya-klyuchnikov/buck,kageiit/buck,SeleniumHQ/buck,clonetwin26/buck,rmaz/buck,SeleniumHQ/buck,LegNeato/buck,zhan-xiong/buck,facebook/buck,brettwooldridge/buck,LegNeato/buck,zhan-xiong/buck,clonetwin26/buck,k21/buck,brettwooldridge/buck,marcinkwiatkowski/buck,zhan-xiong/buck,romanoid/buck,JoelMarcey/buck,brettwooldridge/buck,LegNeato/buck,JoelMarcey/buck,facebook/buck,dsyang/buck,ilya-klyuchnikov/buck,brettwooldridge/buck,zpao/buck,kageiit/buck,marcinkwiatkowski/buck,dsyang/buck,k21/buck,dsyang/buck,robbertvanginkel/buck,rmaz/buck,brettwooldridge/buck,robbertvanginkel/buck,romanoid/buck,zhan-xiong/buck,shs96c/buck,LegNeato/buck,robbertvanginkel/buck,zhan-xiong/buck,shybovycha/buck,rmaz/buck,SeleniumHQ/buck,rmaz/buck,facebook/buck,shybovycha/buck,JoelMarcey/buck,shs96c/buck,k21/buck,ilya-klyuchnikov/buck,SeleniumHQ/buck,clonetwin26/buck,LegNeato/buck,LegNeato/buck,romanoid/buck,robbertvanginkel/buck,SeleniumHQ/buck,nguyentruongtho/buck,Addepar/buck,dsyang/buck,brettwooldridge/buck,dsyang/buck,Addepar/buck,dsyang/buck,marcinkwiatkowski/buck,kageiit/buck,robbertvanginkel/buck,JoelMarcey/buck,romanoid/buck,k21/buck,marcinkwiatkowski/buck,robbertvanginkel/buck,marcinkwiatkowski/buck,shs96c/buck,SeleniumHQ/buck,SeleniumHQ/buck,LegNeato/buck,Addepar/buck,clonetwin26/buck,robbertvanginkel/buck,shybovycha/buck,rmaz/buck,zpao/buck,k21/buck,clonetwin26/buck,ilya-klyuchnikov/buck,Addepar/buck,brettwooldridge/buck,ilya-klyuchnikov/buck,shs96c/buck,clonetwin26/buck,zhan-xiong/buck,romanoid/buck,LegNeato/buck,rmaz/buck,zhan-xiong/buck,rmaz/buck,Addepar/buck,shs96c/buck,zhan-xiong/buck,kageiit/buck,JoelMarcey/buck,facebook/buck,zpao/buck,zhan-xiong/buck,kageiit/buck,shs96c/buck,ilya-klyuchnikov/buck,robbertvanginkel/buck,rmaz/buck,zhan-xiong/buck,shybovycha/buck,dsyang/buck,shs96c/buck,k21/buck,romanoid/buck,nguyentruongtho/buck,shs96c/buck,nguyentruongtho/buck,dsyang/buck,shybovycha/buck,brettwooldridge/buck,Addepar/buck,nguyentruongtho/buck,SeleniumHQ/buck,romanoid/buck,LegNeato/buck,JoelMarcey/buck,clonetwin26/buck,clonetwin26/buck,Addepar/buck,dsyang/buck,brettwooldridge/buck,shs96c/buck,clonetwin26/buck,shybovycha/buck,marcinkwiatkowski/buck,romanoid/buck,SeleniumHQ/buck,dsyang/buck