ci: work around install-qt-action propagating cache:true to setup-python

jurplel/install-qt-action@v4 invokes actions/setup-python internally
with the same `cache:` value the user passes to the Qt action. Our
`cache: true` (intended for Qt's own install cache) gets propagated
verbatim, so setup-python receives `cache: 'true'` — which it rejects
with "Caching for 'true' is not supported" because the only valid
values are 'pip' / 'pipenv' / 'poetry'. Surfaces consistently on
Gitea/Forgejo's act-runner.

Workaround: provide our own actions/setup-python@v5 step (Python is
needed for aqtinstall, the tool install-qt-action uses to fetch Qt),
and set `setup-python: false` on the Qt action so it skips the
broken internal call. Qt's own install cache (the `cache: true` we
actually want) keeps working.

Applied to both .gitea/workflows/ci.yml and .gitea/workflows/release.yml.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -45,12 +45,18 @@ jobs:
         working-directory: framework/php
         run: composer phpunit
 
+      - name: Setup Python (for install-qt-action's aqtinstall)
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.12'
+
       - name: Setup Qt 6
         uses: jurplel/install-qt-action@v4
         with:
           version: '6.5.*'
           modules: 'qtquickcontrols2'
           cache: true
+          setup-python: false
 
       - name: Cache CMake build
         uses: actions/cache@v4

.gitea/workflows/release.yml

@@ -28,12 +28,18 @@ jobs:
         working-directory: framework/php
         run: composer install --no-interaction --prefer-dist
 
+      - name: Setup Python (for install-qt-action's aqtinstall)
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.12'
+
       - name: Setup Qt 6
         uses: jurplel/install-qt-action@v4
         with:
           version: '6.5.*'
           modules: 'qtquickcontrols2'
           cache: true
+          setup-python: false
 
       - name: Install FrankenPHP
         run: |