ci: fix wpt export job (#34640)

The path to `.python-version` used by our setup-python action is
relative to the current working directory, which is not the servo
repo when WPT Export job runs.

Fixes #34639.

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

.github/workflows/pull-request-wpt-export.yml

@@ -30,6 +30,8 @@ jobs:
           token: ${{ secrets.WPT_SYNC_TOKEN }}
       - name: Setup Python
         uses: ./servo/.github/actions/setup-python
+        with:
+          python-version-file-path: ./servo/.python-version
       - name: Install requirements
         run: |
           uv venv