Patch metadata-validation Github action: add python virtualenv

The upstream hardware-metadata-validation action presently fails to install
python dependencies with a permissions error for the pip cache directory.
Until it's fixed upstream, use a virtualenv.

.github/workflows/hardware-metadata-validation.yml

@@ -21,6 +21,13 @@ jobs:
       image: docker.io/zmkfirmware/zmk-dev-arm:3.5
     steps:
       - uses: actions/checkout@v4
+      - name: Setup Python virtual environment
+        run: |
+          apt install python3-venv
+          python3 -m venv /tmp/venv
+          source /tmp/venv/bin/activate
+          echo VIRTUAL_ENV="$VIRTUAL_ENV" >> $GITHUB_ENV
+          echo PATH="$PATH" >> $GITHUB_ENV
       - name: Install dependencies
         run: pip install -r app/scripts/requirements.txt
       - name: West init