Run contributor PR tests on free tier (#25745)

```release-note
NONE
```

.github/workflows/server-test-template.yml

@@ -16,9 +16,23 @@ on:
         type: string
 
 jobs:
+  choose-runner:
+    runs-on: ubuntu-latest
+    outputs:
+      runner: ${{ steps.step1.outputs.runner }}
+    steps:
+      - name: Check branch
+        id: step1
+        run: |
+          if [ ${{ github.event.pull_request.head.repo.full_name == github.event.pull_request.base.repo.full_name }} ]; then
+            echo "runner=ubuntu-latest-8-cores" >> "$GITHUB_OUTPUT"
+          else
+            echo "runner=ubuntu-latest" >> "$GITHUB_OUTPUT"
+          fi
   test:
+    needs: choose-runner
     name: ${{ inputs.name }}
-    runs-on: ubuntu-latest-8-cores
+    runs-on: ${{ needs.choose-runner.outputs.runner }}
     env:
       COMPOSE_PROJECT_NAME: ghactions
       BUILD_IMAGE: mattermost/mattermost-build-server:20230904_golang-1.20.7