aws-cdk: (pipelines) Environment variables placed in Pipeline > Stages > Action > Configuration break deploy

Environment variables were previously placed in AWS::CodeBuildProject > Properties > Environment as a list.

Now it is placed in Pipeline > Stages > Action > Configuration as a string which causes:

1 validation error detected: Value at 'pipeline.stages.2.member.actions.1.member.configuration' failed to satisfy constrain
t: Map value must satisfy constraint: [Member must have length less than or equal to 1000, Member must have length greater than or equal to 1] (Service: AWSCodePipeline; Status Code: 400; Error Code: ValidationException; Request ID: e4
df2712-2f07-45e6-a0bd-9ebf10a0b8c2; Proxy: null)

and I am unable to deploy pipeline.

Reproduction Steps

environment_variables = {name: codebuild.BuildEnvironmentVariable(value=value.value)
                                 for name, value in EnvVars.__members__.items()}

pipeline = pipelines.CdkPipeline(self, "Pipeline",
                                         cloud_assembly_artifact=cloud_assembly_artifact,
                                         pipeline_name="MyPipeline",
                                         source_action=cp_actions.CodeCommitSourceAction(
                                            action_name="GetSourceCode",
                                            output=source_artifact,
                                            repository=repository),
                                         synth_action=pipelines.SimpleSynthAction(
                                            environment={"privileged": True},
                                            environment_variables=environment_variables,
                                            source_artifact=source_artifact,
                                            cloud_assembly_artifact=cloud_assembly_artifact,
                                            install_commands=["npm install -g aws-cdk"],
                                            synth_command="cdk synth")
                                         )

• 1.87.0
• Framework Version:
• node v14.15.1
• Windows 10
• Python 3.8

This is 🐛 Bug Report