gitlab-ci: Manage pacman cache

Drop unused packages from download cache.
Signed-off-by: Jan Vesely <jano.vesely@gmail.com>
2 jobs for pacman-cache in 2 minutes and 45 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #699937
latest-lcms

00:02:45

passed #699938
latest-nolcms

00:02:45