Return full go binary path instead of "go" for standard SDKs

This change fixes problems with running gofmt on my machine.

As a side note, I've never used the Go App Engine SDK and I'm curious to know if that has a different "go" binary. If it does, I'll go ahead and modify my changes in bd3fce4a9ca9582d6d57960bdf860f3eb8c85c29 to use this method instead of just always assuming `GO_BIN_PATH` is usable.