AWS: Using environment variables in CodeBuild buildspec

Source: Using environment variables in AWS CodeBuild buildspec.yml
See S3_BUCKET below, which is first expanded from ImportValue, then used in BuildSpec.

  AvUsageQuarantineBuildProject:
    Type: AWS::CodeBuild::Project
    Condition: Preprod
    Properties:
      Name: av-usage-quarantine-build
      ServiceRole: !GetAtt AvUsageQuarantineBuildProjectRole.Arn
      Artifacts:
        Type: CODEPIPELINE
        Name: av-usage-quarantine
      Environment:
        Type: LINUX_CONTAINER
        ComputeType: BUILD_GENERAL1_SMALL
        Image: aws/codebuild/java:openjdk-8
        EnvironmentVariables:
          - Name: S3_BUCKET
            Value: !ImportValue delivery-AvDeliveryUtilsBucket
      Source:
        Type: CODEPIPELINE
        BuildSpec: !Sub |
          version: 0.2
          phases:
            install:
              commands:
                - cd ~; wget https://raw.github.com/technomancy/leiningen/stable/bin/lein; chmod +x lein; mv lein /bin
                - LEIN_ROOT=true lein
            build:
              commands:
                - cd $CODEBUILD_SRC_DIR
                - LEIN_ROOT=true lein uberjar
            post_build:
              commands:
                - aws cloudformation package --template template.yaml --s3-bucket $S3_BUCKET --s3-prefix lambda --output-template av-usage-quarantine-output-template.yaml