DEV: Fix app-cache regression in tests workflow (#28626)

This regressed in b83a2a34a4 because the
Github actions docs doesn't make it clear that `runner.name` is actually
the runner's name plus some unique string appended at the end. Why they
would do that is beyond me.

.github/workflows/tests.yml

@@ -167,13 +167,18 @@ jobs:
           echo "127.0.0.1 minio.local" | sudo tee -a /etc/hosts
           echo "127.0.0.1 discoursetest.minio.local" | sudo tee -a /etc/hosts
 
+      - name: Get CPU cores
+        id: cpu-info
+        run: echo "cpu-cores=$(nproc)" >> $GITHUB_OUTPUT
+
       - name: Fetch app state cache
         uses: actions/cache@v4
         id: app-cache
         with:
           path: tmp/app-cache
           key: >-
-            ${{ runner.name }}-
+            ${{ runner.os }}-
+            ${{ steps.cpu-info.outputs.cpu-cores }}-
             ${{ hashFiles('.github/workflows/tests.yml') }}-
             ${{ hashFiles('db/**/*', 'plugins/**/db/**/*') }}-
             ${{ hashFiles('config/environments/test.rb') }}-