fix `fdroid build` with non-git repos

HEAD is really only in git.  This was introduced in
a4e43108
2 jobs for collection-fixes-and-KnownVuln in 19 minutes and 55 seconds (queued for 35 minutes and 51 seconds)
Status Job ID Name Coverage
  Test
passed #7072983
test

00:19:55

failed #7043902
test

00:13:46