[Makefile] Change getting github credentials to lazy eval.
[Makefile] Change getting github credentials to lazy eval.

ppamorim/eigen,ashkan18/eigen,Shawn-WangDapeng/eigen,orta/eigen,artsy/eigen,xxclouddd/eigen,ashkan18/eigen,gaurav1981/eigen,ayunav/eigen,srrvnn/eigen,neonichu/eigen,gaurav1981/eigen,orta/eigen,neonichu/eigen,ACChe/eigen,ichu501/eigen,xxclouddd/eigen,foxsofter/eigen,artsy/eigen,artsy/eigen,liduanw/eigen,orta/eigen,neonichu/eigen,ppamorim/eigen,liduanw/eigen,foxsofter/eigen,xxclouddd/eigen,neonichu/eigen,xxclouddd/eigen,artsy/eigen,ACChe/eigen,orta/eigen,ACChe/eigen,artsy/eigen,Havi4/eigen,srrvnn/eigen,srrvnn/eigen,zhuzhengwei/eigen,ichu501/eigen,gaurav1981/eigen,foxsofter/eigen,liduanw/eigen,TribeMedia/eigen,artsy/eigen,1aurabrown/eigen,foxsofter/eigen,Havi4/eigen,neonichu/eigen,ppamorim/eigen,zhuzhengwei/eigen,gaurav1981/eigen,ichu501/eigen,ayunav/eigen,Havi4/eigen,gaurav1981/eigen,1aurabrown/eigen,ayunav/eigen,ayunav/eigen,srrvnn/eigen,1aurabrown/eigen,ashkan18/eigen,Shawn-WangDapeng/eigen,ashkan18/eigen,TribeMedia/eigen,foxsofter/eigen,zhuzhengwei/eigen,ayunav/eigen,ashkan18/eigen,liduanw/eigen,Havi4/eigen,mbogh/eigen,srrvnn/eigen,liduanw/eigen,ichu501/eigen,xxclouddd/eigen,ACChe/eigen,mbogh/eigen,Shawn-WangDapeng/eigen,artsy/eigen,mbogh/eigen,Shawn-WangDapeng/eigen,zhuzhengwei/eigen,mbogh/eigen,ACChe/eigen,TribeMedia/eigen,TribeMedia/eigen,1aurabrown/eigen,TribeMedia/eigen,ppamorim/eigen,ichu501/eigen,Shawn-WangDapeng/eigen