Azure ARM does not support boot diagnostics for non public cloud

_This issue was originally opened by @davemedvitz as hashicorp/packer#9333. It was migrated here as a result of the [Packer plugin split](https://github.com/hashicorp/packer/issues/8610#issuecomment-770034737). The original body of the issue is below._

#### Overview of the Issue

When setting boot_diag_storage_account when also specifying cloud_environment_name, packer throws an error

vhd: Code="DeploymentFailed" Message="At least one resource deployment operation failed. Please list deployment operations for details. Please see https://aka.ms/DeployOperations for usage details." Details=[{"code":"BadRequest","message":"{\r\n \"error\": {\r\n \"code\": \"InvalidParameter\",\r\n \"message\": \"The blob URL's domain must be blob.core.usgovcloudapi.net.\",\r\n \"target\": \"bootDiagnostics.storageAccountUri\"\r\n }\r\n}"}]

### Packer version

Packer version 1.5.6

### Analysis

I did some digging, thinking that I wasn't providing the diag storage account in the right way.

The function SetBootDiag in template_builder.go (line 383 in master @ commit c016f37) hard codes the storage URI suffix.

Start in template_builder.go at SetBootDiag, identified in the issue as hard-coding the storage URI suffix. Trace how cloud_environment_name is provided and how boot diagnostics builds the storage account URI. Done means boot diagnostics uses the correct blob domain for non-public Azure clouds without breaking the public-cloud case.

Written by the indexing model from the issue text.