go: add GOPATH option

[WolfangAukang]

Needed to add this option because on the workplace I have to create a separate path for my Go directory.

I am using null as default value here because as GOPATH must be an absolute path (it complains it if is relative), using $HOME/go as a default value is not a good idea.

[zimbatm]

This is going to create issues as soon as the configuration is shared between users. A better approach would be to set env = [{ name = "GOPATH"; eval = "$HOME/go"; }].

[WolfangAukang]

@zimbatm before pushing my changes and regarding your review, I believe the type for this option should be types.either types.path types.str right? If I leave it only as types.path with $HOME/go as default value, the builds for the PR will fail.