ci(cache): only save caches on master branch

[thomaseizinger]

Description

Previously, we would always save certain smaller caches like clippy etc on each PR. This still amounts to a lot of caches. With this patch, we conditionally only save them when the workflow is run on the master branch. This gives most pull requests a good base to work with to speed them up significantly without us keeping around a lot of caches and have GitHub invalidate them constantly.

On master, the condition evaluates to true which saves the cache: https://github.com/thomaseizinger/rust-libp2p/actions/runs/3699904381/jobs/6267774899#step:5:3
On pull-requests, the condition evaluates to false which skips saving it: https://github.com/thomaseizinger/rust-libp2p/actions/runs/3700055536/jobs/6268096357#step:4:3

Notes

Draft until I confirm this works as expected in my fork. Confirmed that it works.

Links to any relevant issues

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

[jxs]

Makes sense, thanks for this Thomas!

[thomaseizinger]

Blocked from merging by #3252.