semgrep only for non-forked

.github/workflows/semgrep.yml

@@ -8,6 +8,7 @@ on:
 jobs:
   semgrep:
     name: Scan
+    if: "github.repository == 'polkadot-js/phishing'"
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2