Simplifies Makefiles to not use `.GOPATH` symlink.
Updates `test.yaml` to reuse the Go cache in between jobs instead of
removing and downloading all the dependencies for every job.
Changelog: other
Due to a typo, the output of `gitlab-pages -version` was always `dev` instead
of the actual version. The variable name is VERSION, not Version.
Closes https://gitlab.com/gitlab-org/gitlab-pages/issues/140