command: Update backend-unchanged fixture for new expectations

Our serialization of the backend configuration has changed slightly for
Terraform 0.12 due to reimplementing it in terms of the HCL2 types, so
the base case that should be unchanged during the test needs to be
changed.

command/test-fixtures/backend-unchanged/.terraform/terraform.tfstate

@@ -5,9 +5,10 @@
     "backend": {
         "type": "local",
         "config": {
-            "path": "local-state.tfstate"
+            "path": "local-state.tfstate",
+            "workspace_dir": null
         },
-        "hash": 9073424445967744180
+        "hash": 4282859327
     },
     "modules": [
         {