ci: make Cygwin ensurepip available to tests

The Cygwin fast job now reaches the test suite, but test_installation creates another pip-enabled virtual environment. That nested venv failed because Cygwin's patched ensurepip found no wheels in /usr/share/python-wheels after the workflow stopped installing the generic wheel packages.

Install Cygwin's compatible setuptools wheel and put the downloaded, SHA-256-verified pip 26.0.1 wheel in ensurepip's shared wheel directory. Creating the workflow venv normally then verifies the same path used later by the installation test while retaining Python 3.9 coverage.

Validation: git diff --check; Ruby YAML parsing; pip wheel SHA-256 verification; Cygwin package archive inspection.

Author: codex

.github/workflows/cygwin-test.yml

@@ -45,7 +45,7 @@ jobs:
     - name: Install Cygwin
       uses: cygwin/cygwin-install-action@v6
       with:
-        packages: curl git python39
+        packages: curl git python39 python-setuptools-wheel
         add-to-path: false  # No need to change $PATH outside the Cygwin environment.
 
     - name: Arrange for verbose output
@@ -73,11 +73,10 @@ jobs:
 
     - name: Set up virtual environment
       run: |
-        pip_wheel=/tmp/pip-26.0.1-py3-none-any.whl
-        python3.9 -m venv --without-pip .venv
+        pip_wheel=/usr/share/python-wheels/pip-26.0.1-py3-none-any.whl
         curl -fsSLo "$pip_wheel" https://files.pythonhosted.org/packages/de/f0/c81e05b613866b76d2d1066490adf1a3dbc4ee9d9c839961c3fc8a6997af/pip-26.0.1-py3-none-any.whl
         python3.9 -c 'import hashlib, pathlib, sys; assert hashlib.sha256(pathlib.Path(sys.argv[1]).read_bytes()).hexdigest() == sys.argv[2]' "$pip_wheel" bdb1b08f4274833d62c1aa29e20907365a2ceb950410df15fc9521bad440122b
-        PYTHONPATH="$pip_wheel" .venv/bin/python -m pip install "$pip_wheel"
+        python3.9 -m venv .venv
         echo 'BASH_ENV=.venv/bin/activate' >>"$GITHUB_ENV"
 
     - name: Update PyPA packages