chore: improve link checker (#4192)
chore: improve link checker (#4192)

* chore: improve link checker

- only checks README.md
- exclude local links
- exclude mail address
- add exclude regex

fixes #4191

* fix: fix the args

* chore: reduce the concurrency again

* chore: exclude private repo
googleapis/repo-automation-bots,googleapis/repo-automation-bots,googleapis/repo-automation-bots,googleapis/repo-automation-bots,googleapis/repo-automation-bots