fix: always specify agent identity ID when logging into azure for lin…

…ux (#5408)

Co-authored-by: Cameron Meissner <[email protected]>

packer.mk

@@ -61,13 +61,17 @@ endif
 endif
 
 az-login:
-	@echo "Logging into Azure with agent VM MSI..."
+ifeq (${MODE},windowsVhdMode)
 ifeq ($(origin MANAGED_IDENTITY_ID), undefined)
 	@echo "Logging in with Hosted Pool's Default Managed Identity"
 	@az login --identity
 else
 	@echo "Logging in with Hosted Pool's Managed Identity: ${MANAGED_IDENTITY_ID}"
 	@az login --identity --username ${MANAGED_IDENTITY_ID}
+endif
+else
+	@echo "Logging into Azure with identity: ${AZURE_MSI_RESOURCE_STRING}..."
+	@az login --identity --username ${AZURE_MSI_RESOURCE_STRING}
 endif
 	@echo "Using the subscription ${SUBSCRIPTION_ID}"
 	@az account set -s ${SUBSCRIPTION_ID}