.github/workflows/build_wheels.yml: install python-3.12 on windows.

Seems to be necessary at the moment, depending on what runner we get.

Author: julian-smith-artifex-com

.github/workflows/build_wheels.yml

@@ -83,6 +83,21 @@ jobs:
 
       - uses: actions/checkout@v2
 
+      # Get Python 3.12 x32 and x64 on Windows. (As of 2023-10-12 these are not
+      # always available by default.)
+      #
+      - name: Install Python 3.12 x32 on Windows.
+        if: runner.os == 'Windows'
+        uses: actions/setup-python@v2
+        with:
+          python-version: '3.12'
+          architecture: x86
+      - name: Install Python 3.12 x64 on Windows.
+        if: runner.os == 'Windows'
+        uses: actions/setup-python@v2
+        with:
+          python-version: '3.12'
+
       # Get Python for running cibuildwheel. This also ensures that 'python'
       # works on MacOS, where it seems only 'python3' is available by default.
       #