GHA: depends job requires projects to have changes merged on default branch

[rjbou]

The current functioning of the depends job in a PR (from branch named fix-y) is:

1. check if a branch origin/fix-y or user-that-opened-the-PR/fix-y in targeted project exists, and pull it
2. otherwise, pull project default branch
3. try to compile the project on pulled branch

This makes changes on opam lib api on projects need to be merge on default branch once opam PR is merged. The side effect is it then requires to have the project to pin opam master branch to be able to compile, until next opam release.

We should change this behaviour to avoid those requirements.

One solution could be to add an extra step between 1 & 2 to retrieve an arbitrary named branch (eg opam-current), that would contain all opam api changes of opam master's branch, and projects/fix-y shoulbe be opened and merged on that branch.