Improve make (go install) errors

When make runs go install, you can get a compilation error. Before this change the compilation error would print a relative path src/gitlab.com/gitlab-org/gitaly/internal/server/pprof.go:17:1. This error is not going to be "clickable" in IDE's because the path is relative to the _build subdirectory of your Gitaly checkout.

After this change, the go install error will print internal/server/pprof.go:17:1, which is a valid path relative to the project root.