Show differing pytest arguments in step name

In the "Test with pytest" step of the Cygwin test jobs.

This is to distinguish the newly split jobs from each other more
clearly when glancing at their steps for a run.

.github/workflows/cygwin-test.yml

@@ -93,6 +93,6 @@ jobs:
         python --version
         python -c 'import os, sys; print(f"sys.platform={sys.platform!r}, os.name={os.name!r}")'
 
-    - name: Test with pytest
+    - name: Test with pytest (${{ matrix.additional-pytest-args }})
       run: |
         pytest --color=yes -p no:sugar --instafail -vv ${{ matrix.additional-pytest-args }}