Usage of rust-cache triggers warnings about usage of Node.js 16

[carsten-wenderdel]

Currently every usage of this plugin triggers a warning, see for example https://github.com/Swatinem/rust-cache/actions/runs/7772794956/

Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: actions/checkout@v3. For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

Cause is the usage of version 3 of actions/checkout.

As rust-cache already migrated from Node 16 to Node 20 itself, I think it makes sense to upgrade to version 4 of actions/checkout.

I'm going to create a PR for that.