Pin uv-managed Python 3.12 in CI for PyInstaller

The runner's system CPython is built without a shared libpython3.x.so,
which PyInstaller --onefile requires. setup-uv's python-version input
makes uv provision its managed CPython, which ships the shared library.

.gitea/workflows/ci.yml

@@ -12,6 +12,10 @@ jobs:
       - uses: actions/checkout@v4
 
       - uses: astral-sh/setup-uv@v5
+        with:
+          # Pin uv's managed CPython: the runner's system python lacks
+          # libpython3.x.so, which PyInstaller needs for --onefile builds.
+          python-version: "3.12"
 
       - name: Install dependencies
         run: uv sync --frozen
@@ -35,6 +39,10 @@ jobs:
       - uses: actions/checkout@v4
 
       - uses: astral-sh/setup-uv@v5
+        with:
+          # Pin uv's managed CPython: the runner's system python lacks
+          # libpython3.x.so, which PyInstaller needs for --onefile builds.
+          python-version: "3.12"
 
       - name: Install dependencies
         run: uv sync --frozen --group build