disable running nightly build on forks

Author: the-thing

.github/workflows/nightly-build.yml

@@ -10,6 +10,7 @@ on:
 
 jobs:
   release-snapshot:
+    if: github.repository == 'quickfix-j/quickfixj'
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v5