fix(ci): drop unused Set up Python step so Post cleanup stops failing

The dockerized-test workflow never runs pip on the runner — Python is only
invoked inside the Dockerfile build using the base image's own Python. The
'cache: pip' directive on setup-python@v5 therefore ran its Post cleanup
against an empty /home/runner/.cache/pip and failed the job even though all
12 dockerized suite cases passed.

Also bumps actions/checkout to v5 to drop the Node 20 deprecation warning.

Author: maxday

.github/workflows/dockerized-test.yml

@@ -14,14 +14,7 @@ jobs:
   dockerized-test:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
-
-      - name: Set up Python
-        uses: actions/setup-python@v5
-        with:
-          python-version: '3.14'
-          cache: pip
-          cache-dependency-path: requirements/dev.txt
+      - uses: actions/checkout@v5
 
       - name: Build the test image
         run: |