json ci: only run for main repo

Author: serkonda7

.github/workflows/gen_json_grammar.yml

@@ -10,6 +10,7 @@ on:
 
 jobs:
   gen-json-grammars:
+    if: github.repository == 'tiabeast/vscode-vba'
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3