1. Proposed title of this feature request
Allow overriding config with AMTOOL_CONFIG_FILE env var 

2. What is the nature and description of the request?
Allow overriding config with AMTOOL_CONFIG_FILE env var.

Pull request with more details: https://github.com/prometheus/alertmanager/pull/4356

3. Why does the customer need this? (List the business requirements here)
Business reason: when you have multiple clusters, it is useful to be able to target different AlertManager instances from different shell sessions for amtool, similar to what you can do with KUBECONFIG for oc, kubectl, etc.

4. List any affected packages or components.
Alert Manager