Revert "Update build-validation.yml (dotnet#7644)" (dotnet#7652)

This reverts commit 34a2c6c.

Author: gewarren

.github/workflows/build-validation.yml

@@ -5,7 +5,7 @@ name: 'Snippets 5000'
 # events on the main branch only.
 on:
   pull_request:
-    branches: [ turnoff ]
+    branches: [ main ]
     types: [opened, synchronize, reopened]
 
 env: