CI: Explicitly setup Python to build wheels

We currently use any system-installed Python version to invoke the wheel
building process (which then happens inside a container). The exact
Python version does not matter, but it must be a working Python version
-- and that's currently not the case for the macos11 builds: Python and
the Python installation directory are not inside PATH, hence we install
nox, but it's not found in the next step.

actions/runner-images#6518 fixes the problem,
but is not yet rolled out universally. It seems like using setup-python
even for this case is recommended
(actions/runner-images#6507 (comment)),
so let's do that.

.github/workflows/build-test-release.yml

@@ -26,7 +26,9 @@ jobs:
           - macos-11
     steps:
       - uses: actions/checkout@v3
-
+      - uses: actions/setup-python@v4
+        with:
+          python-version: "3.11"
       - name: Install nox
         run: python3 -m pip install nox