Set CodeBuild env var from previous step in codepipeline - aws-cloudformation

Question

I have the following Codepipeline (cloudformation template snippet).
My BuildDeployWebappTest step needs the name of an s3 bucket created in the previous ExecuteChangeSet step.
As you can see BuildDeployWebappTest step uses the WebappCodeBuildProjectTestStage CodeBuild project config. The WEBAPP_S3_BUCKET env var is what needs to be set to the value of the bucket name created in the ExecuteChangeSet step.
How can I make this happen?
Currently I have to create S3 bucket outside of this pipeline and "hard code" the S3 bucket name as a CloudFormation parameter (TestStageS3Bucket) to the pipeline below.
The ExecuteChangeSet produces a CloudFormation output named WebAppBucket. I can export it if needed.
I know CodeBuild supports multiple InputArtifacts, but I don't know how I'd reference a CloudFormation OutputArtifact in the CodeBuild project config OR within the CodeBuild buildspec.yml file itself.
...
WebappCodeBuildProjectTestStage:
Type: AWS::CodeBuild::Project
Properties:
Artifacts:
Type: CODEPIPELINE
Environment:
ComputeType: BUILD_GENERAL1_SMALL
PrivilegedMode: false
Type: LINUX_CONTAINER
Image: !Ref CodeBuildImage
EnvironmentVariables:
- Name: WEBAPP_S3_BUCKET
Value: !Ref TestStageS3Bucket
- Name: APP_STAGE
Value: test
- Name: ANGULAR_BUILD
Value: test
ServiceRole: !Ref CodeBuildRole
Source:
Type: CODEPIPELINE
BuildSpec: !Ref WebappBuildspecPath
TimeoutInMinutes: !Ref BuildTimeout
...
Pipeline:
Type: AWS::CodePipeline::Pipeline
Properties:
ArtifactStore:
Location: !Ref 'ArtifactStoreBucket'
Type: S3
DisableInboundStageTransitions: []
Name: !Ref AWS::StackName
RoleArn: !GetAtt PipelineRole.Arn
Stages:
- Name: Source
Actions:
- Name: Source
ActionTypeId:
Category: Source
Owner: ThirdParty
Provider: GitHub
Version: '1'
OutputArtifacts:
- Name: MyAppCode
Configuration:
Owner: !Ref GithubOrg
Repo: !Select [ 0, !Split [ '--', !Ref 'AWS::StackName' ] ]
PollForSourceChanges: false
Branch: !Select [ 1, !Split [ '--', !Ref 'AWS::StackName' ] ]
OAuthToken: !Ref GithubOAuthToken
RunOrder: 1
- Name: DeployTestResources
Actions:
- Name: CreateChangeSet
ActionTypeId:
Category: Deploy
Owner: AWS
Provider: CloudFormation
Version: '1'
InputArtifacts:
- Name: MyAppCode
Configuration:
ActionMode: CHANGE_SET_REPLACE
RoleArn: !GetAtt CFNRole.Arn
Capabilities: CAPABILITY_IAM
StackName: !Sub "${AWS::StackName}--test--gen"
ChangeSetName: !Sub "${AWS::StackName}--test--changeset"
TemplatePath: MyAppCode::aws/cloudformation/template.yml
TemplateConfiguration: !Sub "MyAppCode::${TestCloudFormationTemplateParameters}"
RunOrder: 1
- Name: ExecuteChangeSet
ActionTypeId:
Category: Deploy
Owner: AWS
Provider: CloudFormation
Version: '1'
Configuration:
ActionMode: CHANGE_SET_EXECUTE
RoleArn: !GetAtt CFNRole.Arn
StackName: !Sub "${AWS::StackName}--test--gen"
ChangeSetName: !Sub "${AWS::StackName}--test--changeset"
OutputFileName: TestOutput.json
OutputArtifacts:
- Name: DeployTestResourcesOutput
RunOrder: 2
- Name: BuildDeployWebappTest
Actions:
- Name: CodeBuild
InputArtifacts:
- Name: MyAppCode
ActionTypeId:
Category: Build
Owner: AWS
Provider: CodeBuild
Version: '1'
Configuration:
ProjectName: !Ref WebappCodeBuildProjectTestStage
RunOrder: 1