only run nightly for main repo (#1407)

Signed-off-by: Pierre Tessier <pierre@pierretessier.com>
Co-authored-by: Juliano Costa <julianocosta89@outlook.com>

Author: puckpuck

.github/workflows/nightly-release.yml

@@ -10,6 +10,7 @@ on:
 jobs:
   build_and_push_images:
     uses: ./.github/workflows/build-images.yml
+    if: github.repository == 'open-telemetry/opentelemetry-demo'
     with:
       push: true
       version: nightly-${{ github.run_id }}