ci: split the build_on_fedora job up into a template and a real job

Let's use a template for the generic case of "building on Fedora"
and a named job for the specific job itself.

The real change here is the artifact handling: by default the artifacts
collected for a .build job are only the meson build logs.

The build_on_fedora job collects the installed files as well (to be used in
the pages job later).