simplified -- according to regular "ghci" script;

--- a/lib/Tools/ghci	Thu Nov 08 14:48:20 2018 +0100
+++ b/lib/Tools/ghci	Thu Nov 08 14:56:53 2018 +0100
@@ -4,9 +4,4 @@
 #
 # DESCRIPTION: invoke GHC interaction within the Isabelle environment
 
-if [ -d "$ISABELLE_STACK_ROOT" -a -n "$ISABELLE_GHC" ]; then
-  isabelle_stack ghci "$@"
-else
-  echo "Cannot execute ghci: missing Isabelle GHC setup" >&2
-  exit 127
-fi
+isabelle ghc --interactive "$@"