Ensure datadog upload is skipped for forks (#76238)

Follow-up to #73180 ensures we
don't run this for forks which was dropped in the PR.

x-ref:
https://github.com/vercel/next.js/actions/runs/13422276429/job/37497537420?pr=75784

.github/workflows/build_reusable.yml

@@ -244,7 +244,7 @@ jobs:
           path: packages/next/dist/compiled/next-server/report.*.html
 
       - name: Upload test report to datadog
-        if: ${{ inputs.afterBuild && always() }}
+        if: ${{ inputs.afterBuild && always() && !github.event.pull_request.head.repo.fork }}
         run: |
           # Add a `test.type` tag to distinguish between turbopack and next.js runs
           # Add a `nextjs.test_session.name` tag to help identify the job