Enable pull_request trigger for nightly builds

Enable nightly build workflow for pull requests.

Author: svekars

.github/workflows/build-tutorials-nightly.yml

@@ -15,7 +15,7 @@ name: Build tutorials (nightly/test)
 # download the binaries in .jenkins/build.sh.
 on:
   # Only main branch for now. Uncomment the below line to enable it on PRs.
-  # pull_request:
+  pull_request:
 
   # Comment out the below line to disable on the main branch
   push: