buildRustCrate: don't swallow build.rs output

This is executed curing the build, and usually quite helpful in telling
you how the build failed, if it failed.

For example, when linking against system libraries, it shows which
system lib could not be discovered, the pkg-config path used etc.

Not showing it simply removes all this, and trying to parse out the
"important" bits and only show these is probably also brittle.

The output generally isn't that spammy normally, so let's just `tee` it.

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