ci: disable cache in semver-checks

[galargh]

Description

I noticed that semver-checks cache artifacts are tiny (hundreds of kilobytes tops) so it might worth maintaining them. Additionally, I hope it might help with the network issue reported here: #3782 (comment). It's a shot in the dark but some solutions to similar reported problems include clearing directories - my thinking is, maybe we're caching something that we shouldn't.

Notes & open questions

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates

[thomaseizinger]

These cache artifacts save us over a minute on each job so I'd like to retain them! :)

They represent the rustdoc JSON for the released versions on crates.io. I would be silly to rebuild them on every run.