ci: install git in gruppenpruefung too (same alpine/git gap)

gruppenpruefung.py runs 'git log --all' over the sibling clones (line 108) but
its job used python:3.12-alpine with no before_script, so it would hit the same
FileNotFoundError: 'git' as validate did. Dormant only because the job runs on
schedule/web, not push. Add 'apk add git'. The job's API/CA and GITLAB_TOKEN
prerequisites remain tracked separately (management#31).

.gitlab-ci.yml

@@ -62,6 +62,8 @@ gruppenpruefung:
   rules:
     - if: $CI_PIPELINE_SOURCE == "schedule"
     - if: $CI_PIPELINE_SOURCE == "web"
+  before_script:
+    - apk add --no-cache git >/dev/null   # gruppenpruefung.py ruft 'git log' auf
   script:
     - |
       if [ -z "$GITLAB_TOKEN" ]; then