cli: prevent panic on `operator debug` (#14992)

If the API returns an error during debug bundle collection the CLI was
expanding the wrong error object, resulting in a panic since `err` is
`nil`.

.changelog/14992.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+cli: prevent a panic when the Nomad API returns an error while collecting a debug bundle
+```

command/operator_debug.go

@@ -1404,7 +1404,7 @@ func writeResponseStreamOrErrorToFile[T any](obj []T, apiErr error, getWriterFn
 	defer writer.Close()
 
 	if apiErr != nil {
-		wrapped := errorWrapper{Error: err.Error()}
+		wrapped := errorWrapper{Error: apiErr.Error()}
 		return writeJSON(wrapped, writer)
 	}