[ARM Builder] Build KeyVault Secret not deleted when `build_key_vault_name` is specified

Whenever a user specifies a `build_key_vault_name` in their template we do not make a call to delete this key vault, this makes sense but as a result we leave behind an unused secret in the vault.

To solve this we need to make a call to delete the secret when a key vault is provided, if the key vault isn't provided it gets deleted correctly.

Start in the ARM Builder cleanup path and trace how cleanup behaves when build_key_vault_name is specified versus omitted. Confirm the existing behavior for the no-name case, then verify that the secret is deleted while the user-provided key vault remains.

Written by the indexing model from the issue text.