ci: Use Swatinem/rust-cache@v2 to cache

[tottoto]

Motivation

Resolving dependencies for the MSRV build in CI takes time.

Solution

Uses Swatinem/rust-cache@v2 to cache dependencies.

[jplatte]

I forgot last time, can you also add

with:
  save-if: ${{ github.ref == 'refs/heads/main' }}

? That way cache effectiveness is a tad worse for PRs that change / update dependencies, but cache hit ratio is much better (caches from PRs can't get re-used in other PRs).

[tottoto]

Updated to add it.