No ability to use github env vars in manifest

[chlorophant]

As far as I can tell there is no way to use a github env var or secret inside the copilot manifest using this action.

This functionality is supported by copilot when using it directly.

Example manifest.yml:

image:
 build: Dockerfile
  port: 8080
  args:
    GIT_SSH_PRIVATE_KEY: ${env.GIT_SSH_PRIVATE_KEY}

This is essential for many use cases