ci: Note how many processes Meson is using

The CI runners on gitlab.gnome.org seem to have huge numbers of CPUs
but they don't have the IO capacity to match, leading to test timeouts.
1 job for sam/test-fixes in 3 minutes and 41 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #432426
test-fedora-latest

00:03:41