Set user agent for lychee (alshedivat#2368)

See: alshedivat#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'