Revisit installing pip in venv in separate step

The previous commit pinned the python39-pip package, but it may
still be upgraded when installed automatically in the
`python -m venv` step, since that step runs something like
`python3.9 -Im ensurepip --upgrade --default-pip`.

This replays 4605dd6 to install pip in a separate step. This
differs from the solution in gitpython-developers#2007 in that this does not use the
bootstrap script.

Author: EliahKagan

.github/workflows/cygwin-test.yml

@@ -57,9 +57,13 @@ jobs:
 
     - name: Set up virtualenv
       run: |
-        python3.9 -m venv .venv
+        python3.9 -m venv --without-pip .venv
         echo 'BASH_ENV=.venv/bin/activate' >>"$GITHUB_ENV"
 
+    - name: Install pip in virtualenv
+      run: |
+        python -m ensurepip
+
     - name: Update PyPA packages
       run: |
         # Get the latest pip, wheel, and prior to Python 3.12, setuptools.