gitlab-ci: rework how release pipelines work

Previously, releases would run all jobs and only enable the upload jobs for the were relevant. Instead, make it so that all jobs share an idea of what the tag is for and skip jobs not related to the release at hand.

This is because we now have a number of jobs that are quite expensive (namely the gfortran and qt builds) and running them for both the merge to master for some change and the release that is probably happening right after is quite wasteful (unless it is one of their releases of course).

Additionally, factor out the release tag check into its own variable. This allows a single set of rules for the entire project.