shell_prefix: physical /bin/env on Cygwin;

--- a/src/Pure/Tools/isabelle_system.scala	Sat Aug 23 19:42:15 2008 +0200
+++ b/src/Pure/Tools/isabelle_system.scala	Sat Aug 23 19:42:16 2008 +0200
@@ -77,7 +77,7 @@
 
   def shell_prefix() = {
     if (Pattern.matches(".*-cygwin", getenv_strict("ML_PLATFORM")))
-      Some(platform_path("/usr/bin/env"))
+      Some(platform_path("/bin/env"))
     else None
   }