CloudFormation Sub into a string from Map

CFN-SubStringMapping.yaml

# Used to provide map values to a long string, e.g. EnvironmentVariables to CodeBuild.

Parameters:
  Environment:
    Description: Environment to deploy
    Type: String
    Default: prod
    AllowedValues:
      - prod
      - stage
Mappings: 
  Environments:
    prod:  
      accountName: aws-prod
      tfStateBucket: aws-prod1-terraform-tfstate
    stage:
      accountName: aws-stage
      tfStateBucket: aws-stage-terraform-tfstate
 Resources:
  Blah:
    Properties:
      EnvironmentVariables: 
        !Sub 
        - |
          [{"name":"TF_URL","value": "https://relea..."},
          {"name":"tf_statebucket", "value": "${LocalFindInMapBucket}"},
        - { LocalFindInMapBucket: !FindInMap [Environments, !Ref Environment, tfStateBucket] }