ci: linkcheck should retry

[avivace]

The linkcheck should retry at least 5 times with a delay of 5-10 seconds. Lately the CI is failing a lot because of false positives on links that are actually online.

[pinobatch]

Would it help to do the retry breadth-first? Fetch all URLs once, then try all that failed, then try all that failed again. This ordering builds the time it takes to check the other URLs into the delay.

Also would it help to cache live/dead status of a URL across multiple runs of CI within a week? That might help with web servers that impose rate limits on the CI server's IP address.