Try new strategy for staleness check branch selection

.github/workflows/staleness_check.yml

@@ -23,13 +23,8 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        branch: [main, 25.x, 27.x, 28.x]
         os: [{ name: Linux, value: ubuntu-latest}]
-        exclude:
-          # If we are in a presubmit run, only test main
-          - branch: ${{ !inputs.continuous-run && '25.x' }}
-          - branch: ${{ !inputs.continuous-run && '27.x' }}
-          - branch: ${{ !inputs.continuous-run && '28.x' }}
+        branch: ${{ inputs.continuous-run && ['main'] || ['main', '25.x', '27.x', '28.x'] }}
 
     name: Test staleness ${{ matrix.os.name }} ${{ matrix.branch }}
     runs-on: ${{ matrix.os.value }}