gitlab-ci: Move artifact preparation to separate script

gitlab-ci: Move artifact preparation to separate script

It's currently only needed for the meson-main and meson-arm64 jobs, not
the other meson build jobs.

Also remove MESON_SHADERDB, just run .gitlab-ci/run-shader-db.sh
directly from the meson-main job.

v2:
* Also run prepare-artifacts.sh in meson-arm64 script
v3:
* Move tarball creation into the new script as well, as it prevented
  ccache --show-stats from running in after_script

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com> # v1
Reviewed-by: Eric Anholt <eric@anholt.net>