CI: There's no need to run unit tests/coverage for all crates if a PR touches only a specific crate and *nothing* workspace-wide

[epilys]

While CI runs don't take long, it feels kind of wasteful to run tests for the entire workspace when we have binary crates (e.g. no other crate depends on each vhost device crate). Plus flakey test failures from other crates can affect an unrelated PR. We could generate the test pipelines in a different way compared to other rust-vmm repos. However this is low-hanging fruit and not important or urgent.