flowey: add a request for installing/getting bsdtar

[jstarks]

Rather than having a bsdtar_name freefloating function, this should probably be backed by a ctx.reqv(install_bsdtar::Request), which return a PathBuf to the bsdtar executable.

That way, we can evolve the underlying install_bsdtar node to install the dependency on platforms where it might not be present in-box in the future.

In the future, I'd also want to hook that into flowey's as-yet-unrealized-and-not-yet-designed "platform requirements" API, where I'd like nodes to be able to communicate requests to the pipeline resolver in order to have certain dependencies pre-installed on the job runner (similar to ADO's "demands" system they have for pools)

Originally posted by @daprilik in https://github.com/microsoft/openvmm/pull/122#discussion_r1806807755