chore: run lychee link checker on PRs only (#934)

The Lychee link checking action seems to be failing on `master` (perhaps
due to insufficient permissions)?
We need to run the checks on PRs only anyway.

Author: barjin

.github/workflows/check.yaml

@@ -105,6 +105,7 @@ jobs:
 
             - name: Run Lychee Link Checker
               id: lychee
+              if: github.event_name == 'pull_request'
               uses: lycheeverse/lychee-action@v2.8.0
               with:
                   fail: false