ci(spread): only run core24 spread tests

The intention here is to speed-up spread runs and make the development
loop faster. I've run the non-core24-tests in this feature branch and
can confirm that they pass (or have unrelated failures).

.github/workflows/spread.yml

@@ -38,8 +38,9 @@ jobs:
       fail-fast: false
       matrix:
         spread-jobs:
-          - google:ubuntu-20.04-64
-          - google:ubuntu-22.04-64
+          # Only run core24 tests for now; fix this once all those tests are OK
+          - google:ubuntu-20.04-64:tests/spread/core24/
+          - google:ubuntu-22.04-64:tests/spread/core24/
 
     steps:
       - name: Checkout snapcraft