Take "advantage" of CI environment changes

The mini variant now has everything needed to roll a source tarball.
It also has git installed so we don't need a convoluted find for our
style check anymore.

.gitlab-ci.yml

@@ -14,10 +14,10 @@ stages:
   - archive
 
 prepare:
-  image: $REGISTRY_HUB:debian-9-full
+  image: $REGISTRY_HUB:debian-9-mini
   stage: prepare
   script:
-  - find -path ./.git -prune -o -type f | xargs tools/style-check.sh
+  - git ls-files | xargs ./tools/style-check.sh
   - ./autogen.sh
   - ./tools/create-changelog.sh
   - ./tools/update-upstreams.sh