gitlab-ci: Consolidate pacman setup and cache downloaded pacman packages

Signed-off-by: Jan Vesely <jano.vesely@gmail.com>
4 jobs for master in 2 minutes and 37 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #295694
latest-autotools-lcms

00:02:07

passed #295695
latest-autotools-nolcms

00:02:36

passed #295692
latest-meson-lcms

00:01:43

passed #295693
latest-meson-nolcms

00:01:15