ci: Limit link checks to intel/perfmon

Automatically checking links is intended to be a low volume reminder to
update stale links. It isn't necessary for every fork (examples in [1]
and [2]) to also act as a reminder. This pull request checks if the
repository is 'intel/perfmon' and further removes push and pull_request
triggers.

[1] https://github.com/edwarddavidbaker/perfmon/actions/workflows/check-markdown-links.yml?query=event%3Aschedule
[2] https://github.com/captain5050/perfmon/actions/workflows/check-markdown-links.yml?query=event%3Aschedule

.github/workflows/check-markdown-links.yml

@@ -1,19 +1,16 @@
 name: Check Markdown links
 
 on:
-  pull_request:
-    branches:
-      - main
-  push:
-    branches:
-      - main
   schedule:
     # Tuesdays at 9AM PST. GitHub Actions run in UTC.
     - cron: '0 16 * * 2'
 
 jobs:
   markdown-link-check:
     runs-on: ubuntu-latest
+    # Only run link checks in the main repo and not forks. The intent
+    # is a low volume reminder to update stale links.
+    if: github.repository == 'intel/perfmon'
     steps:
     - uses: actions/checkout@v3
     - uses: gaurav-nelson/github-action-markdown-link-check@v1