add pull request event

Author: dasormeter

.github/workflows/blackduck.yml

@@ -3,7 +3,11 @@ defaults:
   run:
     shell: bash -euo pipefail -c "source nix.source && source .envrc.vars && exec bash {0}"
 on:
-  push:
+  pull_request:
+    events:
+      - opened
+      - synchronize
+      - reopened
 #    paths:
 #      - go.mod
 #      - go.sum