Add caching for GitHub Actions

[orhun]

Right now the servers are being built for each workflow run which takes time. It would be nice to e.g. cache target folder for Rust and zig-cache for the Zig server.

[kassane]

Maybe help:
https://github.com/Hanaasagi/zig-action-cache

[orhun]

I was thinking of using actions/cache to specify the directories to cache and restore them somehow. However, I haven't got around to look into that.