run linux tests using buildrecall

[kcking]

This is an exploratory PR to demonstrate using buildrecall to run the linux CI tests.

Here's an example of an incremental run of the synth-cargo-test action https://github.com/buildrecall/synth/actions/runs/1382870650, note the windows pathway is unchanged.

Buildrecall's configuration can be found in buildrecall.toml. Note that an API key is required for brr to run in the getsynth github org.

We're happy to help speed up other parts of the CI or your local dev using buildrecall as well!