proper local host (amending 62d7ef1da441);

--- a/src/Pure/General/ssh.scala	Wed Nov 08 21:45:02 2023 +0100
+++ b/src/Pure/General/ssh.scala	Thu Nov 09 11:30:33 2023 +0100
@@ -432,7 +432,7 @@
     port: Int = 0,
     user: String = ""
   ): System = {
-    if (host.isEmpty) Local
+    if (is_local(host)) Local
     else open_session(options, host = host, port = port, user = user)
   }