Lychee action consistently fails with GitHub links

[0x009922]

The Lychee action (https://github.com/lycheeverse/lychee-action) we use in pr.yml consistently fails to verify a single link from GitHub:

• [429] https://github.com/AppImage/AppImageSpec/blob/ce1910e6443357e3406a40d458f78ba3f34293b8/draft.md#type-2-image-format | Network error: Too Many Requests

I tried (in #5399):

• Specifying GITHUB_TOKEN as they suggest - didn't help to reduce rate limiting
• Specifying larger --retry-wait-time (5 seconds instead of 1) - didn't help

Finally, I just set fail: false for this action to not fail the entire workflow.

[aoyako]

Caching can be added to the workflow to reduce the issue. https://github.com/lycheeverse/lychee-action/?tab=readme-ov-file#utilising-the-cache-feature

However, StackOverflow links tend to return 403, in this case:

• use https://lychee.cli.rs/troubleshooting/redirects/#bot-protection
• ignore StackOverflow links (exclude in config)