fix: add runs-on to README.md example (#460)

This is needed in order for this example to "work". The runner that
needs to be picked is specified above, but it's still handy if we can
simply copy / paste from the README.

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README.md

@@ -49,6 +49,9 @@ support](https://cloud.google.com/support).**
 ```yaml
 jobs:
   job_id:
+    # Any runner supporting Node 20 or newer
+    runs-on: ubuntu-latest
+
     # Add "id-token" with the intended permissions.
     permissions:
       contents: 'read'