Pin pip with pip (Cygwin package doesn't pin)

This tries with an older pip, but if the problem is the build
rather than the version, then it would also help.

Author: EliahKagan

.github/workflows/cygwin-test.yml

@@ -30,7 +30,7 @@ jobs:
     - name: Install Cygwin
       uses: cygwin/cygwin-install-action@v4
       with:
-        packages: python39 python39-pip=23.0.1-1 python39-virtualenv git
+        packages: python39 python39-virtualenv git
         add-to-path: false  # No need to change $PATH outside the Cygwin environment.
 
     - name: Arrange for verbose output
@@ -55,6 +55,10 @@ jobs:
         # and cause subsequent tests to fail
         cat test/fixtures/.gitconfig >> ~/.gitconfig
 
+    - name: Pin pip
+      run: |
+        python -m pip install pip==23.3.1
+
     - name: Install project and test dependencies
       run: |
         python -m pip install ".[test]"