Run schedule CI more often due to link cache

[marxin]

This attempt runs CI job more often so that the chunk of expired links to github.com is relatively small and so it's going to prevent: 429 Too Many Requests error.

CC: @camelid @Kobzol

[camelid]

Thanks! I wonder if it would be better to just make the cache last longer instead though. Any thoughts on that? On a related note, do you know if the cache also includes links that were in the previous cache?

[marxin]

Oh, I finally understand what's wrong with the cache reuse:

Failed to save: Unable to reserve cache with key linkcheck--0.8.1, another job may be creating this cache. More details: Cache already exists. Scope: refs/heads/master, Key: linkcheck--0.8.1, Version: 33f8fd511bed9c91c40778bc5c27cb58425caa894ab50f9c5705d83cb78660e0
Warning: Cache save failed.

Let me prepare a separate PR, where I am going to fix it.

[marxin]

Fix: #2223