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Uppercase variable looked more official, and so - to me - was confusing.

Fixes https://github.com/matthew-brett/multibuild/issues/259.

See:
https://github.com/matthew-brett/multibuild/pull/242#issuecomment-513244208

Docker crosses filesystems.

Zip output corrupts the output for returning the output directory.

To get pip's platform.

We can't easily predict what the output directory will be for a Github
release archive.  Get fetch_unpack to tell us.  Add some tests that
won't fail, but at least show us the output.

Previous version was using a sub-shell to execute, and therefore not
catching errors during execution of the functions that had errors, but
did not return non-0 exit codes.

Add tests.

See: https://github.com/matthew-brett/multibuild/issues/166

Thanks to Andrew Murray for pointing out this bug.

See: https://github.com/pypa/manylinux/issues/160