(pipelines): ECS task execution role is not given access to the env file

I am deploying a ECS app that uses an Asset Environment File.
The ECS task is failing to start because the task execution role is not getting access to the CDK asset bucket, where the environment file is uploaded to.

Things to note:
- The deployment is cross-region, but in the same account
- There was no Asset stage when I did the initial `cdk deploy` of my pipeline - it was created by the first run of the Self Mutate stage (because the pipeline has multiple sources).

Granting access to the asset bucket explicitly (by importing it by name) solves the issue, but it is a dirty hack.

I am afraid I do not have a complete minimal example, but maybe my description would be enough.

Let me know if it is not - I will try to think of something.

### Environment

- **CDK CLI Version :** 1.115
- **Framework Version:** 1.115
- **Node.js Version:** 16.5.0
- **OS :** Linux
- **Language (Version):** all

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This is :bug: Bug Report