[GitHub][Workflows] Prevent multiple private email comments (temporarily) (llvm#80648)

Seems the easiest way to quiet this workflow while we figure out the final form of it.

Author: DavidSpickett

.github/workflows/email-check.yaml

@@ -1,6 +1,9 @@
 name: "Check for private emails used in PRs"
 
-on: pull_request_target
+on:
+  pull_request_target:
+    types:
+      - opened
 
 permissions:
   contents: read