Fix CI due to GH Actions regression (missing Firefox)

GitHub has rolled out Ubuntu 22 as the new default
but this is still missing Firefox, causing the following
failure for all jobs:

> Starting browser FirefoxHeadless
> ERROR [launcher]: Cannot start FirefoxHeadless
> Can not find the binary `firefox`

Ref actions/runner-images#5490

.github/workflows/test.yml

@@ -8,7 +8,7 @@ env:
 jobs:
   run:
     name: Node ${{ matrix.node }}
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     strategy:
       fail-fast: false