fix(ci): testground workflow hanging indefinitely (#8741)

* ci: set timeout on testground job
* ci: use testground action which exits early on scheduling failures

Note: this will be continued in #8731

.github/workflows/testground-on-push.yml

@@ -25,7 +25,9 @@ jobs:
     steps:
       - uses: actions/checkout@v2
       - name: testground run
-        uses: coryschwartz/[email protected]
+        # restore the mainline once https://github.com/coryschwartz/testground-github-action/pull/2 is released
+        uses: galargh/testground-github-action@6bdc2d4f12280a3e0ec654fd75fe170d4b7931df
+        timeout-minutes: 5
         with:
           backend_addr: ${{ matrix.backend_addr }}
           backend_proto: ${{ matrix.backend_proto }}