azure-arm builder does not allow subscription_id selection when using managed identity authentication

[ghost]

This issue was originally opened by @dougbw as hashicorp/packer#9140. It was migrated here as a result of the Packer plugin split. The original body of the issue is below.

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Overview of the Issue

Currently it is impossible to specify a subscription_id when using managed identity authentication, as when only subscription_id is provided packer assumes an interactive login is desired.

In our case we need the base images to be stored in a different subscription to on running our CI/CD pipeline build agents. A single managed identity can have access to many Azure subscriptions.

Reproduction Steps

• Create a user assigned managed identity
• Grant the identity Azure RBAC permissions to multiple subscriptions
• Assign the managed identity to a vm
• On the vm execute az login --identity and you should see output for multiple subscriptions
• Provide a subscription_id to the azure-arm builder in packer and you will be stuck on an interactive login