ci: use setup-python install so codspeed builds flamegraphs correctly (#5997)

Author: davidhewitt

.github/workflows/benches.yml

@@ -20,9 +20,14 @@ jobs:
       - uses: actions/checkout@v6.0.2
       - uses: astral-sh/setup-uv@v7
         with:
-          # codspeed action needs to be run from within the final Python environment
-          activate-environment: true
           save-cache: ${{ github.ref == 'refs/heads/main' || contains(github.event.pull_request.labels.*.name, 'CI-save-pr-cache') }}
+
+      # Using this action is still necessary for CodSpeed to build flamegraphs correctly,
+      # see note about setup-python in https://codspeed.io/docs/benchmarks/python#recipes
+      - uses: actions/setup-python@v6
+        with:
+          python-version: ${{ env.UV_PYTHON }}
+
       - uses: dtolnay/rust-toolchain@stable
         with:
           components: rust-src