Add the possibility to publish from a different branch than main

[Guillaumebeuzeboc]

Currently when we enable pushing it only does if we are on main: https://github.com/ubuntu-robotics/snap_workflows/blob/main/.github/workflows/snap.yaml#L109

It would be convenient to handle other cases for repositories using different branches for different versions (like core20, core22 or monolithic, multisnap)