ci: install git in the validate job so pruefe_prosa can run

pruefe_prosa.py shells out to 'git cat-file' to verify cited commit SHAs, but
the python:3.12-alpine image has no git and before_script only installed pyyaml.
The job crashed with FileNotFoundError on every push since the migration
(pipelines #255, #257). Add 'apk add git' to before_script. Verified green in
the same image locally: validate, gen_status --check, upstream_drift and
pruefe_prosa all pass.

.gitlab-ci.yml

@@ -43,6 +43,7 @@ validate:
   rules:
     - if: $CI_PIPELINE_SOURCE == "push"
   before_script:
+    - apk add --no-cache git >/dev/null   # pruefe_prosa.py braucht git cat-file
     - pip install --quiet pyyaml
   script:
     - python3 scripts/validate.py