Set user agent for lychee (#2368)

See: alshedivat/al-folio#2366
See: lycheeverse/lychee#1411

.github/workflows/broken-links.yml

@@ -49,4 +49,4 @@ jobs:
         with:
           fail: true
           # removed md files that include liquid tags
-          args: --exclude-path README.md --exclude-path _pages/404.md --exclude-path _pages/blog.md --exclude-path _posts/2018-12-22-distill.md --verbose --no-progress './**/*.md' './**/*.html'
+          args: --user-agent 'curl/7.54' --exclude-path README.md --exclude-path _pages/404.md --exclude-path _pages/blog.md --exclude-path _posts/2018-12-22-distill.md --verbose --no-progress './**/*.md' './**/*.html'