ci: pin the Cygwin pip wheel

Cygwin test (fast) and test (perf) now fail while creating the virtual environment because ensurepip exits unsuccessfully. The failure began when the mirror advanced python-pip-wheel from 26.0.1-1 to 26.2.1-1 and occurs under both Python 3.9 and 3.12.

Pin the last wheel release known to pass while retaining Python 3.12. Cygwin setup supports package=version selections, and the current mirror still advertises this exact release as a previous version.

Validated with git diff --check and Ruby YAML parsing.

Author: codex

.github/workflows/cygwin-test.yml

@@ -45,7 +45,7 @@ jobs:
     - name: Install Cygwin
       uses: cygwin/cygwin-install-action@v6
       with:
-        packages: git python312 python-pip-wheel python-setuptools-wheel python-wheel-wheel
+        packages: git python312 python-pip-wheel=26.0.1-1 python-setuptools-wheel python-wheel-wheel
         add-to-path: false  # No need to change $PATH outside the Cygwin environment.
 
     - name: Arrange for verbose output