Stop the job at some point if it is for a forked PR

Co-authored-by: Tom Prince <tom.prince@hocat.ca>

.circleci/config.yml

@@ -180,6 +180,14 @@ jobs:
               --arg collectCoverage true \
               --attr doc
 
+      - run:
+          name: Early return if this build is from a forked PR
+          command: |
+            if [ -n "$CIRCLE_PR_NUMBER" ]; then
+              echo "Nothing to do for forked PRs, so marking this step successful"
+              circleci step halt
+            fi
+            
       - run:
           name: "Push to Cachix"
           when: "always"