support COMPOSER env var in validate command (#6834)
support COMPOSER env var in validate command (#6834)

* support COMPOSER env var in validate command

nicolas-grekas/composer,localheinz/composer,floverdevel/composer,vdm-io/composer,slbmeh/composer,andrerom/composer,stof/composer,fabpot/composer,dzuelke/composer,pierredup/composer,stefangr/composer,staabm/composer,milleruk/composer,gencer/composer,johnstevenson/composer,erasys/composer,composer/composer,naderman/composer,alcohol/composer,SpacePossum/composer