Pin MacOS version in GH actions (#17183)

Fix failing MacOS tests in CI

Python 3.9 is not available on the latest MacOS images
actions/setup-python#850

---------

Co-authored-by: Marc Mueller <30130371+cdce8p@users.noreply.github.com>

.github/workflows/test.yml

@@ -74,7 +74,8 @@ jobs:
         - name: mypyc runtime tests with py39-macos
           python: '3.9.18'
           arch: x64
-          os: macos-latest
+          # TODO: macos-13 is the last one to support Python 3.9, change it to macos-latest when updating the Python version
+          os: macos-13
           toxenv: py
           tox_extra_args: "-n 2 mypyc/test/test_run.py mypyc/test/test_external.py"
         - name: mypyc runtime tests with py38-debug-build-ubuntu