Gerrit: cmd/gobind: add GOROOT to cmd.Env from ctx.GOROOT

Gobind generates language bindings that make it possible to call Go functions from Java and Objective-C. Gobind also supports accessing API from Java or Objective C from Go.

In two places gobind launches go commands but does not pass along the system’s GOROOT causing the subprocess go’s to look in /usr/local/go and fail if GOROOT is not there.