ci: add rust-cache action

[Firestar99]

Maybe it'll speed up our CI?
https://github.com/Swatinem/rust-cache

My investigation log

Adding caching

First run (with no cache present) is looking real bad:

• windows test +1m 8s
• windows difftest +6m 24s
• no idea why the cache would take so long when there's nothing to download and nothing has been compiled???
• all "install rust toolchain" are now 0s, so we probably have to move them before the cache action

First CI with restore

Comparing latest main to this PR:

• overall time: 27m 39s -> 16m 8s
• longest job "windows-difftest": 27m 29s -> 16m 0s
• cache restore, measured from seconded empty commit in this PR, which may have been removed:
  • windows: 58s to 2m 3s
    • first run of windows difftest has an obscene "restore" of 6m 24s even though there's nothing to restore???
  • linux: ~30s
  • mac: 30s - 40s
• cache creation, measured from first commit in this PR:
  • windows: 30s - 1m
  • linux: 8s - 15s
  • mac: 30s - 50s

Install toolchain before restore

• rust-cache seems to install the toolchain for you via rustup if you don't do it yourself, toolchain install has 0s execution time
• Action: move it before cache restore
• Results: old rust-cache time is now split between rustup install and new rust-cache
• Could caches contain rustup builds? Try purging caches

[nazar-pc]

In my projects I usually disable caching on Windows. Their gzip or whatever compression they use is so spectacularly slow that it is faster to download/install things from scratch than decompressing/recompressing cache.