Update so that the prototype build isn't run on trunk

I've realized that even with the dependency on the block job, the prototype job would run after merging PRs to trunk

Author: iangmaia

.buildkite/pipeline.yml

@@ -91,6 +91,7 @@ steps:
     blocked_state: passed
 
   - label: 🏗️ Prototype Build
+    if: build.pull_request.id != null
     depends_on:
       - run-on-demand-prototype-build
     command: .buildkite/commands/prototype-build.sh