Add valgrind jobs

This adds manual valgrind jobs for core, base, good, bad and ugly.

Some day we might want to run them by default, but maybe not quite yet.

This still has some rough edges, but since it's manual anyway (and works) I think we should get it merged and then improve on it incrementally.

Ideally we'd get valgrind and debuginfo for glib2 and glibc installed on the fedora image by default too, as the dnf steps cause repo updates and that takes eternities for some reason (on all runners).

• gstreamer ca. 5-6 minutes (on 32 cores)
• gst-plugins-base ca. 4-5 minutes (on 32 cores)
• gst-plugins-good ca. 5-6 minutes (on 32 cores)
• gst-plugins-bad ca. 7-8 minutes (on 32 cores)
• gst-plugins-ugly ca. 30 seconds (on 32 cores)