mach: recreate .venv when .python-version changes (#34712)

The new images published in servo/ci-runners#12 should have `uv`
installed already and the initial build of servo triggered during the
base image construction will force the installation of the Python
version mentioned at the time of the image construction (3.12). When
.python-version changes, we can no longer use the .venv baked into the
image and must recreate the it to avoid activating the environment.

Signed-off-by: Mukilan Thiyagarajan <mukilan@igalia.com>

.github/workflows/mac-wpt.yml

@@ -46,14 +46,14 @@ jobs:
       - name: Prep test environment
         run: |
           gtar -xzf target.tar.gz
-          python3 ./mach bootstrap --skip-lints
+          ./mach bootstrap --skip-lints
       - name: Smoketest
-        run: python3 ./mach smoketest --${{ inputs.profile }}
+        run: ./mach smoketest --${{ inputs.profile }}
       - name: Run tests
         run: |
           mkdir -p wpt-filtered-logs/macos-${{ inputs.wpt-layout }}
           mkdir -p wpt-full-logs/macos-${{ inputs.wpt-layout }}
-          python3 ./mach test-wpt $WPT_COMMAND_LINE_ARG \
+          ./mach test-wpt $WPT_COMMAND_LINE_ARG \
             --${{ inputs.profile }} --processes $(sysctl -n hw.logicalcpu) --timeout-multiplier 8 \
             --total-chunks ${{ env.max_chunk_id }} --this-chunk ${{ matrix.chunk_id }} \
             --log-raw wpt-full-logs/macos-${{ inputs.wpt-layout }}/${{ matrix.chunk_id }}.log \