Cache toolchain, dependencies in PR/CI

Rust builds seem to be taking longer lately, presumably to set up older toolchains the agents may not have since we target older versions by design. But caching certain directories can help a lot. I use the following in nearly all my Rust repos and it makes a significant improvement in build time.

https://github.com/heaths/akv-cli-rs/blob/241c9ccc84be7ba277c5483192ba3f0714215c64/.github/workflows/ci.yml#L36-L45

```yaml
- name: Cache
uses: actions/cache@v4
with:
path: |
~/.cargo/bin/
~/.cargo/registry/index/
~/.cargo/registry/cache/
~/.cargo/git/db/
target/
key: ${{ runner.os }}-${{ runner.arch }}-cargo-${{ hashFiles('**/Cargo.lock') }}
```

Using the right cache key - including the toolchain (version), platform, and a hash of `Cargo.lock` - we can handle all the variations we have.