Use the real brief_id
Use the real brief_id

Patching the data_api_client's `.get_brief()` method means
that it doesn't matter what URL we call -- the brief we created with
id `1234` still gets returned.
Might as well fix it.

AusDTO/dto-digitalmarketplace-buyer-frontend,AusDTO/dto-digitalmarketplace-buyer-frontend,AusDTO/dto-digitalmarketplace-buyer-frontend,alphagov/digitalmarketplace-buyer-frontend,alphagov/digitalmarketplace-buyer-frontend,alphagov/digitalmarketplace-buyer-frontend,alphagov/digitalmarketplace-buyer-frontend,AusDTO/dto-digitalmarketplace-buyer-frontend