Only check links in changed files for fail-fast workflow (#6850)

Fix #6573

Co-authored-by: Damien Mathieu <42@dmathieu.com>

.github/workflows/links-fail-fast.yml

@@ -10,8 +10,32 @@ on:
 permissions: read-all
 
 jobs:
+  changedfiles:
+    name: changed files
+    runs-on: ubuntu-latest
+    env:
+      PR_HEAD: ${{ github.event.pull_request.head.sha }}
+    outputs:
+      files: ${{ steps.changes.outputs.files }}
+    steps:
+      - name: Checkout Repo
+        uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        with:
+          fetch-depth: 0
+      - name: Get changed files
+        id: changes
+        run: |
+          files=$(git diff --name-only --diff-filter=ACMRTUXB $(git merge-base origin/main $PR_HEAD) $PR_HEAD | xargs)
+
+          # Used for debugging.
+          echo "files=$files"
+
+          echo "files=$files" >> $GITHUB_OUTPUT
+
   check-links:
     runs-on: ubuntu-latest
+    needs: changedfiles
+    if: ${{needs.changedfiles.outputs.files}}
     permissions:
       contents: read
     steps:
@@ -31,7 +55,8 @@ jobs:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         with:
           fail: true
-          args: --max-concurrency 5 --cache --max-cache-age 1d --cache-exclude-status 300..=599 .
+          failIfEmpty: false
+          args: --max-concurrency 5 --cache --max-cache-age 1d --cache-exclude-status 300..=599 ${{needs.changedfiles.outputs.files}}
 
       - name: Save lychee cache
         if: always()