fetchRepoProject: Fix the GnuPG verification

The verification was failing with the following error:
gpg: keyblock resource '/tmp/nix-build-XYZ.drv-0/.repo/repo/./.repoconfig/gnupg/pubring.kbx': No such file or directory

Using an absolute path for $HOME fixes this.

And since 175ecbab the dependencies on
"git" and "gnupg" aren't required anymore as "gitRepo" already covers
them.