Replace local gitlab-ci include with ci-scripts

.gitlab-ci.yml: Replace the local include of a gitlab-ci description (and the accompanying script) with an include of archlinux/ci-scripts. Copy potentially existing keys in script target instead of before_script target. This way we get to remove before_script and only need to specify which script from ci-scripts we are calling in the scripts target, instead of redoing all of before_script. Remove after_script target, as it is taken care of in ci-scripts.

Implements #8 (closed)