Pin actions/setup-python to v4 to dodge corrupted v5 cache on runner

The Gitea Actions runner has a corrupted cache for
actions/setup-python@v5: the cloned worktree has unstaged changes and
the runner can't pull refs/heads/v5 cleanly. As a result the cached
dist/setup/index.js is missing and the step fails before any of our
lint commands run. Pin to v4 (different cache key) so the runner
clones a fresh, unmodified copy.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -23,7 +23,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
-      - uses: actions/setup-python@v5
+      - uses: actions/setup-python@v4
         with:
           python-version: "3.12"
       - run: pip install ruff
@@ -37,7 +37,7 @@ jobs:
     continue-on-error: true
     steps:
       - uses: actions/checkout@v4
-      - uses: actions/setup-python@v5
+      - uses: actions/setup-python@v4
         with:
           python-version: "3.12"
       - name: Install system dependencies
@@ -79,7 +79,7 @@ jobs:
       CARTSNITCH_FERNET_KEY: wXWQsC0FZlhSz2t_tfVQjNUSP8vgAGG3o3pkjrX8Bw0=
     steps:
       - uses: actions/checkout@v4
-      - uses: actions/setup-python@v5
+      - uses: actions/setup-python@v4
         with:
           python-version: "3.12"
       - name: Install system dependencies