gitlab-ci: fix schedule-issuebot job for new pip version

I ported this job from Alpine to Debian to make future upgrades easier and
in sync with all the other jobs.  The Alpine job isn't notably faster than
the debian:bookworm-slim jobs anyway.

.gitlab-ci.yml

@@ -262,7 +262,7 @@ trigger-issuebot:
 schedule-issuebot:
   stage: test
   needs: []
-  image: alpine
+  image: debian:bookworm-slim
   only:
     variables:
       - $SCHEDULE_ISSUEBOT
@@ -277,9 +277,8 @@ schedule-issuebot:
   script:
     - mkdir logs
     - export | grep -F CI_ | grep -vFi -e password -e token > logs/export.txt
-    - apk add --no-cache bash curl ca-certificates python3
-    - python3 -m ensurepip
-    - $pip install python-gitlab
+    - apt-get update
+    - apt-get -qy install --no-install-recommends bash curl ca-certificates python3-gitlab
     - ./tools/schedule-issuebot.py
 
 checkupdates_runner: