renable github actions for linux (facebook#1135)

Summary:
Pull Request resolved: facebook#1135

Renable mononoke linux github actions

Differential Revision: D82818583

Author: ahornby

.github/workflows/mononoke_linux.yml

@@ -3,7 +3,12 @@
 name: Mononoke Linux
 
 on:
-  workflow_dispatch
+  push:
+    branches:
+    - main
+  pull_request:
+    branches:
+    - main
 
 permissions:
   contents: read  #  to fetch code (actions/checkout)