github/workflows: Use Python 3.11 for unix settrace jobs.

GitHub Actions has updated ubuntu-latest to 24.04, which now defaults
CPython to 3.12, which has a known regression with settrace.  Fix that by
explicitly using CPython 3.11.

Signed-off-by: Damien George <damien@micropython.org>

.github/workflows/ports_unix.yml

@@ -173,6 +173,11 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v4
+    - uses: actions/setup-python@v5
+      # Python 3.12 is the default for ubuntu-24.04, but that has compatibility issues with settrace tests.
+      # Can remove this step when ubuntu-latest uses a more recent Python 3.x as the default.
+      with:
+        python-version: '3.11'
     - name: Build
       run: source tools/ci.sh && ci_unix_settrace_build
     - name: Run main test suite
@@ -185,6 +190,11 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v4
+    - uses: actions/setup-python@v5
+      # Python 3.12 is the default for ubuntu-24.04, but that has compatibility issues with settrace tests.
+      # Can remove this step when ubuntu-latest uses a more recent Python 3.x as the default.
+      with:
+        python-version: '3.11'
     - name: Build
       run: source tools/ci.sh && ci_unix_settrace_stackless_build
     - name: Run main test suite