cmd/gb: verify the Go install has not changed
cmd/gb: verify the Go install has not changed

Update #319

hyper-carrot/gb,tbruyelle/gb,tbruyelle/gb,ches/gb,mopemope/gb,hyper-carrot/gb,mopemope/gb,davidkaya/gb,mopemope/gb,lparkes/gb,davidkaya/gb,inconshreveable/gb,constabulary/gb,constabulary/gb,constabulary/gb,tianon/gb,tianon/gb,ches/gb,inconshreveable/gb