CI: switch to the meson-build.sh helper script
This script does the same thing but has a few more toggles than our custom script which means we can set everything we need in the gitlab-ci.yml file and have it DTRT.
For reference: https://gitlab.freedesktop.org/whot/meson-helper
Motivation here is to have the X server's CI run driver builds to ensure we didn't break any headers, see !1318 (merged)
Edited by Peter Hutterer