if building directly in git repo, use file path for remote

if building directly in git repo, use file path for remote

When a git repo has a .fdroid.yml file in it, and `fdroid build` is run
directly in that git repo, then this uses the file path as the remote for
the git repo in build/appid that is actually built.  That makes it possible
to run builds of commits that are only local, and makes things a whole lot
faster.