Run the link checkers on the open source repo as well (#17789)

Author: JamesMGreene

.github/workflows/link-check-dotcom.yml

@@ -3,6 +3,9 @@ name: 'Link Checker: Dotcom'
 on:
   workflow_dispatch:
   push:
+    branches:
+      - main
+  pull_request:
 
 jobs:
   see_if_should_skip:

.github/workflows/link-check-ghae.yml

@@ -3,6 +3,9 @@ name: 'Link Checker: GitHub AE'
 on:
   workflow_dispatch:
   push:
+    branches:
+      - main
+  pull_request:
 
 jobs:
   see_if_should_skip:

.github/workflows/link-check-ghes.yml

@@ -3,6 +3,9 @@ name: 'Link Checker: Enterprise Server'
 on:
   workflow_dispatch:
   push:
+    branches:
+      - main
+  pull_request:
 
 jobs:
   see_if_should_skip: