Fix test failing in Active Storage's GCS service

[maleblond]

Summary

Ref. #42509 (comment)

This PR should fix two tests in main that are failing since #42509: https://buildkite.com/rails/rails/builds/78375#6fb6fd5c-1d74-4491-b9c8-97ee7ad52c67

Error:
--
ActiveStorage::Service::GCSServiceTest#test_upload_with_cache_control:
NoMethodError: undefined method `delete' for nil:NilClass
/rails/activestorage/test/service/gcs_service_test.rb:132:in `ensure in block in <class:GCSServiceTest>'
/rails/activestorage/test/service/gcs_service_test.rb:132:in `block in <class:GCSServiceTest>'

We were trying to use a variable that doesn't exist (service_config instead of SERVICE_CONFIGURATIONS). Those tests only run in the main branch, which is why this failure wasn't caught before the PR was merged

[pixeltrix]

@maleblond thanks for following this up 👍🏻

[pixeltrix]

Ugh, I missed the second commit - for future reference, please squash those out, thanks.