Green-again and faster ci

This fixes CI to green again after recent registry.gitlab.gnome.org/gnome/gnome-runtime-images/gnome:master change that starts to run builds as non-root user - gnome-runtime-images!16 (merged) and dnf commands start to fail.

Note that I'm completely not sure that the method proposed in this MR adopting to non-root build is better than for example switching checks to use another docker images, so it is only a proof-of-concept and a-variant-for-discussion. Also runnung windows builds in parallel with flatpak speeds up things, but makes per-stage-green-or-red statuses less informartive. So fell free to open discussions or even take this as a starting point and fix CI some other way.

Also there may be a problem with lint stage - while it is green - it's fine, but I didn't test that "--show-diff-on-failure" still works correctly on red status since git is not explicitely installed.