I am looking to setup a manual approval before the production build can be triggered from staging.
Is there any workaround to setup manual approval if Google doesn't support it natively?
AWS Manual Approval Action : https://docs.aws.amazon.com/codepipeline/latest/userguide/approvals-action-add.html
Unfortunately there is no equivalent of AWS Manual Approval Action for Cloud Build, however, you can integrate Cloud Build with a 3rd party tool to implement a deployment approval for different enviroments. Here is an article that uses Spinnaker for that.
You could also open a Feature Request so that an implementation of a native approval for deployment can be considered by Google's Product Team by following this link.