6912: Default to host platform for cargo metadata r=jonhoo a=jonhoo
This modifies the logic for calling cargo metadata so that it will use
the host platform if no explicit target platform is given. This is
needed since cargo metadata defaults to outputting information for _all_
targets.
Fixes#6908.
Co-authored-by: Jon Gjengset <jongje@amazon.com>
Co-authored-by: Jon Gjengset <jon@thesquareplanet.com>
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