Optimisation: narrow GitHub Actions CI on Windows to test-under-investigation

Ref: sphinx-doc#14133 (comment)

Author: jayaddison

.github/workflows/main.yml

@@ -185,7 +185,7 @@ jobs:
     - name: Install dependencies
       run: uv pip install . --group test
     - name: Test with pytest
-      run: python -m pytest -vv --durations 25
+      run: python -m pytest -vv --durations 25 tests/test_builders/test_build_linkcheck.py
       env:
         PYTHONWARNINGS: "error"  # treat all warnings as errors