BUILD-9956 fix post run aws auth

[julien-carsique-sonarsource]

BUILD-9956

Add backend input option to force the selection of the backend (github or s3) instead of the default selection based on the repository visibility.

Add tests to cover both github and s3 backends.

Fix the AWS authentication.

• the aws-auth step must share the AWS credentials with runs-on/cache
• using global env is not possible as it pollutes the caller environment
• using step level env fails in the post run execution due to bugs in the AWS client used by runs-on/cache
  The solution is to use aws-actions/configure-aws-credentials to locally save the credentials.

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