CI: Don't run Windows tests on GHA, it takes too much disk space

The GHA job output already croaked about this, like:

    You are running out of disk space. The runner will stop working when
    the machine runs out of disk space. Free space left: 0 MB

.github/workflows/tests.yaml

@@ -49,8 +49,8 @@ jobs:
     - name: Run tests
       # FIXME: Can this be run without elevated privileges?
       run: |
-        sudo $(command -v pytest)
-        coverage xml
+        sudo --preserve-env $(command -v pytest)
+        coverage xml --omit postroj/winrunner.py
 
     - name: Upload coverage results to Codecov
       uses: codecov/codecov-action@v3

testing/racker/test_run_windows.py

@@ -14,6 +14,10 @@
 if "rackerhost-debian11" in socket.gethostname():
     pytest.skip("Nested virtualization with VT-x fails on developer's macOS workstation", allow_module_level=True)
 
+if "GITHUB_ACTIONS" in os.environ:
+    pytest.skip("Installing the Vagrant filesystem image for Windows "
+                "takes too much disk space on GitHub Actions", allow_module_level=True)
+
 
 def run_racker(command: str) -> subprocess.CompletedProcess:
     program_path = Path(sys.argv[0]).parent