Unbreak CI on user branches

Right now CI is totally broken on user branches, at least when any of the farms are disabled. This is because the rules added to skip the pipeline when we're disabling farms, falsely match when user branches are pushed: when pushing a new branch, GitLab sees .ci-farms-disabled/igalia being created (since all files are new) and decides to skip the container jobs entirely. As the build jobs are completely different, they have their own set of rules, so even though the intention of the current rules would result in all user branches having zero jobs (because of the false trigger), the reality is that the pipeline doesn't even parse, since the build jobs are created even though the container jobs are not.

This MR reorganises the container and build-job rules to be very similar (in particular having the when: never nukes be first), makes the comparison a bit more resilient, and then avoids nuking pipelines for user refs.

cc @eric