fix(aws-ecs): use executionRole for event rule target

[SoManyHs]

Fixes #2015.

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Pull Request Checklist

• Testing
  • Unit test added (prefer not to modify an existing test, otherwise, it's probably a breaking change)
  • CLI change?: coordinate update of integration tests with team
  • cdk-init template change?: coordinated update of integration tests with team
• Docs
  • jsdocs: All public APIs documented
  • README: README and/or documentation topic updated
• Title and Description
  • Change type: title prefixed with fix, feat will appear in changelog
  • Title: use lower-case and doesn't end with a period
  • Breaking?: last paragraph: "BREAKING CHANGE: <describe what changed + link for details>"
  • Issues: Indicate issues fixed via: "Fixes #xxx" or "Closes #xxx"
• Sensitive Modules (requires 2 PR approvers)
  • IAM Policy Document (in @aws-cdk/aws-iam)
  • EC2 Security Groups and ACLs (in @aws-cdk/aws-ec2)
  • Grant APIs (only if not based on official documentation with a reference)

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license.

[rix0rrr]

Failed because of NuGet bug:

@aws-cdk/region-info: /usr/share/dotnet/sdk/2.1.402/NuGet.targets(498,5): error : Could not find a part of the path '/tmp/NuGetScratch/d08baa5f-5eee-4c7a-81d6-3240834eb03b'. [/tmp/jsii-pacmak-codeauvqEg/Amazon.CDK.RegionInfo/Amazon.CDK.RegionInfo.csproj]