Fix link checker false positives on root-relative asset links

lychee cannot resolve links like /assets/img/foo.png without being told
the repo root, so any changed file containing a pre-existing root-relative
link fails the check even though the link is valid. Pass --root-dir . so
lychee resolves these against the repo root.

Observed in PR #926, which failed on an unrelated pre-existing image link.

Author: rrrutledge

.github/workflows/link-checker-prs.yml

@@ -39,7 +39,7 @@ jobs:
         name: Link Checker
         uses: lycheeverse/lychee-action@v2
         with:
-          args: --verbose --no-progress --cache --max-cache-age 1d $MARKDOWN_FILES
+          args: --verbose --no-progress --cache --max-cache-age 1d --root-dir . $MARKDOWN_FILES
           fail: true
           failIfEmpty: false
           jobSummary: true