ci: add cache to gitlab ci

Signed-off-by: Filipe Laíns <lains@archlinux.org>
Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>

.gitlab-ci.yml

 image: archlinux
 
+cache:
+  key: system-v1
+  paths:
+    # For some reason Gitlab CI only supports storing cache/artifacts in a path relative to the build directory
+    - .pkg-cache
+
 before_script:
-  - pacman -Syu --noconfirm --noprogressbar --needed
+  - pacman -Syu --noconfirm --noprogressbar --needed --cachedir .pkg-cache
            base-devel git gpgme protobuf pyalpm python-mysql-connector
            python-pygit2 python-srcinfo python-bleach python-markdown
            python-sqlalchemy python-alembic python-pytest python-werkzeug