Bootstrap pip in Cygwin Python 3.12 venv in a separate step

This works around a problem in Python 3.12 on Cygwin where `pip`
fails to intall in a venv via `ensurepip` (both implicitly when the
venv is created normally, and explicitly when `python -m ensurepip`
is attempted), by creating the venv with `--without-pip` and then,
once the venv is set up, bootstrapping `pip` in it by running the
https://bootstrap.pypa.io/get-pip.py script.

Eventually, when `pip` is working automatically in a venv in
Python 3.12 on Cygwin (the problem is specific to that combination,
and the Cygwin `python312*` packages are still marked "test"), this
special-casing can be removed.

.github/workflows/cygwin-test.yml

@@ -12,10 +12,10 @@ jobs:
         include:
         - python-version: "3.9"
           python-cygwin: python39
-          python-command: python3.9
+          venv-args: ''
         - python-version: "3.12"
           python-cygwin: python312
-          python-command: python3.12
+          venv-args: --without-pip
 
       fail-fast: false
 
@@ -40,7 +40,7 @@ jobs:
     - name: Set up Cygwin
       uses: egor-tensin/setup-cygwin@v4
       with:
-        packages: ${{ matrix.python-cygwin }} ${{ matrix.python-cygwin }}-pip ${{ matrix.python-cygwin }}-virtualenv git
+        packages: ${{ matrix.python-cygwin }} ${{ matrix.python-cygwin }}-pip ${{ matrix.python-cygwin }}-virtualenv git wget
 
     - name: Arrange for verbose output
       run: |
@@ -67,9 +67,14 @@ jobs:
 
     - name: Set up virtualenv
       run: |
-        ${{ matrix.python-command }} -m venv .venv
+        python${{ matrix.python-version }} -m venv ${{ matrix.venv-args }} .venv
         echo 'BASH_ENV=.venv/bin/activate' >>"$GITHUB_ENV"
 
+    - name: Bootstrap pip in virtualenv
+      if: contains(matrix.venv-args, '--without-pip')
+      run: |
+        wget -qO- https://bootstrap.pypa.io/get-pip.py | python
+
     - name: Update PyPA packages
       run: |
         # Get the latest pip, wheel, and prior to Python 3.12, setuptools.