ecr: grant methods cause circular references in generated templates

[otterley]

The grant* methods on the ecr.Repository class incorrectly add self-references to the repository's policies, leading to a circular reference error.

Example code:

const repo = new ecr.Repository(this, 'TestHarnessRepo', {
    repositoryName: props.repoName,
});
repo.grantPull(principal);

Rendered template (in relevant part):

TestHarnessRepoAA7E9724:
    Type: AWS::ECR::Repository
    Properties:
      RepositoryName: codebuild-inspec-test-harness
      RepositoryPolicyText:
        Statement:
          - Action:
              - ecr:BatchCheckLayerAvailability
              - ecr:GetDownloadUrlForLayer
              - ecr:BatchGetImage
            Effect: Allow
            Principal:
              Service:
                Fn::Join:
                  - ""
                  - - codebuild.
                    - Ref: AWS::URLSuffix
            Resource:
              Fn::GetAtt:
                - TestHarnessRepoAA7E9724 # !!!! this is the problem
                - Arn

ECR repository policies do not require resources to be defined (or they can be set to *) -- see the documentation for examples.

[otterley]

A workaround for the moment is to use the addToResourcePolicy method on the repo instead.