Pin CodSpeed benchmark job to Python 3.12

Author: saulshanabrook

.github/workflows/CI.yml

@@ -58,7 +58,8 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@v6
         with:
-          python-version-file: ".python-version"
+          # Work around import-time SciPy/sklearn segfaults on the codspeed ARM64 Python 3.13 runner.
+          python-version: "3.12"
       - uses: astral-sh/setup-uv@v7
         with:
           enable-cache: true