Find a build cache somewhere in a parent directory.

def find_config():
    """Find a build cache somewhere in a parent directory."""
    previous = ""
    current = os.getcwd()
    while previous != current:
        check_path = os.path.join(current, "build.cache")
        if os.path.isfile(check_path):
            return check_path
        else:
            previous = current
            current = os.path.dirname(current)
    raise Exception("Can't find build cache")