Upgrade to actions/cache@v4

v2 no longer works.

.github/workflows/test-pyodide.yml

@@ -20,7 +20,7 @@ jobs:
         cache: 'pip'
         cache-dependency-path: '**/setup.py'
     - name: Cache Playwright browsers
-      uses: actions/cache@v2
+      uses: actions/cache@v4
       with:
         path: ~/.cache/ms-playwright/
         key: ${{ runner.os }}-browsers