Wrap CNVkit commands ensuring we use local temporary directories.

def _prep_cmd(cmd, tx_out_file):
    """Wrap CNVkit commands ensuring we use local temporary directories.
    """
    cmd = " ".join(cmd) if isinstance(cmd, (list, tuple)) else cmd
    return "export TMPDIR=%s && %s" % (os.path.dirname(tx_out_file), cmd)