Allow cross-util run commands to be executed in QEMU

[joe-p]

Checklist

• I've looked through the issues and pull requests for similar request
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Describe your request

#1280 introduces the ability to run arbitrary commands in the cross container. This is particularly useful for custom build tools like maturin. What would be even more useful is the ability to run commands in the QEMU system.

Describe why this would be a good inclusion for cross

This would enable cross-util run to be useful for both build steps (i.e. maturin) and running tests (i.e. pytest) within the target system. The main benefit is simplification of tooling for complex build/test workflows. For example, cross could be used for building and testing FFI languages without needing something like https://github.com/uraimo/run-on-arch-action