debian/salsa-ci.yml: Replace except: with rules: and Enable build arm64 on the default branch

Replacing except with rules is needed to have a better control to e.g. override some jobs.

The build arm64 is only enabled by default on the default branch. In other branches, it runs manually and it is allowed to fail, to avoid blocking MRs pipelines.

Draft: could be removed once the project has configured an arm runner.