How to use the blobfile.join function in blobfile

os.environ["CIBW_ENVIRONMENT"] = (
                os.environ["CIBW_ENVIRONMENT"]
                + " TRAVIS_TAG=" + os.environ["TRAVIS_TAG"]
            )
    elif platform.system() == "Windows":
        init_vsvars()

    run("pip install cibuildwheel==1.0.0")
    run("cibuildwheel --output-dir wheelhouse")

    if have_credentials:
        print("upload wheels", platform.system())
        input_dir = "wheelhouse"
        output_dir = f"gs://{GCS_BUCKET}/builds/"
        for filename in bf.listdir(input_dir):
            src = bf.join(input_dir, filename)
            dst = bf.join(output_dir, filename)
            print(src, "=>", dst)
            bf.copy(src, dst, overwrite=True)

def cache_folder(name, dirpath, options, build_fn):
    if os.path.exists(dirpath):
        print(f"cache for {name} found locally")
        return
    
    options_hash = hashlib.md5("|".join(options).encode("utf8")).hexdigest()
    cache_path = bf.join(f"gs://{GCS_BUCKET}", "cache", f"{name}-{options_hash}.tar")
    if "GOOGLE_APPLICATION_CREDENTIALS" not in os.environ:
        # we don't have any credentials to do the caching, always build in this case
        print(f"building without cache for {name}")
        start = time.time()
        build_fn()
        print(f"build elapsed {time.time() - start}")
    elif bf.exists(cache_path):
        print(f"downloading cache for {name}: {cache_path}")
        start = time.time()
        with bf.BlobFile(cache_path, "rb") as f:
            with tarfile.open(fileobj=f, mode="r") as tf:
                tf.extractall()
        print(f"download elapsed {time.time() - start}")
    else:
        print(f"building cache for {name}")
        start = time.time()

os.environ["CIBW_ENVIRONMENT"]
                + " TRAVIS_TAG=" + os.environ["TRAVIS_TAG"]
            )
    elif platform.system() == "Windows":
        init_vsvars()

    run("pip install cibuildwheel==1.0.0")
    run("cibuildwheel --output-dir wheelhouse")

    if have_credentials:
        print("upload wheels", platform.system())
        input_dir = "wheelhouse"
        output_dir = f"gs://{GCS_BUCKET}/builds/"
        for filename in bf.listdir(input_dir):
            src = bf.join(input_dir, filename)
            dst = bf.join(output_dir, filename)
            print(src, "=>", dst)
            bf.copy(src, dst, overwrite=True)