workflows: Compile with Ubuntu 18.04's GLib, but use 2.60 to run tests
workflows: Compile with Ubuntu 18.04's GLib, but use 2.60 to run tests

This avoids a race condition in versions older than 2.60, while still
verifying that we can compile successfully with GLib 2.56.

Not having GLib 2.60 means we can't compile libmalcontent on Ubuntu 18.04,
so move the libmalcontent dependency to the main build job (on Ubuntu
22.04, which is new enough). This also means we don't have to compile
it from source every time.

Signed-off-by: Simon McVittie <0e1aea601907c1a85c2dc96e5ea37e23c58d2e9b@collabora.com>

flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak