pmb.helpers: Automatically migrate fetch and push URL (MR 2429)

This could be re-purposed in the future in case we migrate URLs again.

Co-authored-by: Caleb Connolly <caleb@postmarketos.org>
Tweaked-by: Oliver Smith <ollieparanoid@postmarketos.org>
(cherry picked from commit 344d6f22)
[luca: backport to 2.3.x, add "2.x" suffix to workdir]