Interesting, maybe there's a problem with the encoding, can you please give this a try?

import os

# Retrieve the JSON credentials from Databricks Secrets
cred = dbutils.secrets.get("bigquery-cred", "project-id-name")

# Write the JSON to a temporary file
temp_cred_path = "/tmp/cred.json"
with open(temp_cred_path, "w") as temp_cred_file:
    temp_cred_file.write(cred)

# Set the environment variable to point to the JSON file
os.environ['GOOGLE_APPLICATION_CREDENTIALS'] = temp_cred_path

# Verify the environment variable and file content (for debugging)
print("GOOGLE_APPLICATION_CREDENTIALS:", os.environ['GOOGLE_APPLICATION_CREDENTIALS'])
with open(temp_cred_path, "r") as temp_cred_file:
    print(temp_cred_file.read())