fix(ci): add user agent to link check lychee action

[hanabi1224]

Summary of changes

We got 401 in https://github.com/ChainSafe/forest/actions/runs/14167543185/job/39683880232

• [401] https://blog.ipfs.tech/2023-rust-libp2p-based-ipfs-bootstrap-node/#ipfs-public-dht-bootstrap-nodes | Network error: Unauthorized

This PR adds user agent to the action as suggested in lycheeverse/lychee-action#282 (comment)

A manual run is green: https://github.com/ChainSafe/forest/actions/runs/14169942430/job/39691311962

Changes introduced in this pull request:

Reference issue to close (if applicable)

Closes

Other information and links

Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation. All new code adheres to the team's documentation standards,
• I have added tests that prove my fix is effective or that my feature works (if possible),
• I have made sure the CHANGELOG is up-to-date. All user-facing changes should be reflected in this document.

[LesnyRumcajs]

Adding a user agent gets around the bot protection? I had more expectations around it. 😆