Keep Horreum branch fixed while backporting

[lampajr]

Describe the bug

Right now while backporting changes to the stable branch, e.g., 0.12.x, and given the actual merge strategy we hit the issue that the backporting PR tries to change the HORREUM_BRANCH in the Makefile to the master one.

Example of PR: #7 (ignore the latest commit I added to revert that change)

To Reproduce

Backport a pull request.

Version

From main branch

Proposal

Either we change the merge / cherry-pick strategy or we create a simple script that sets the HORREUM_BRANCH based on the actual backporting target.