environment: keep original HOME and PATH (required for Cygwin integrity);

--- a/src/Pure/System/isabelle_system.scala	Thu Jun 18 15:08:57 2009 +0200
+++ b/src/Pure/System/isabelle_system.scala	Thu Jun 18 17:22:41 2009 +0200
@@ -98,7 +98,9 @@
           if (i <= 0) (entry -> "")
           else (entry.substring(0, i) -> entry.substring(i + 1))
         }
-      Map(entries: _*)
+      Map(entries: _*) +
+        ("HOME" -> java.lang.System.getenv("HOME")) +
+        ("PATH" -> java.lang.System.getenv("PATH"))
     }
     finally { dump.delete }
   }