emacs package setup hook: Fix Fix

A trailing separator in emacs load paths has semantics

cc @Ericson2314 @adisbladis https://github.com/NixOS/nixpkgs/pull/73287

This partially reverts commit ca782498a9

pkgs/build-support/emacs/setup-hook.sh

@@ -1,6 +1,8 @@
 addEmacsVars () {
-  if [[ -d "$1/share/emacs/site-lisp" ]]; then
-      export EMACSLOADPATH="$1/share/emacs/site-lisp${EMACSLOADPATH:+:}${EMACSLOADPATH-}"
+  if test -d $1/share/emacs/site-lisp; then
+      # it turns out, that the trailing : is actually required
+      # see https://www.gnu.org/software/emacs/manual/html_node/elisp/Library-Search.html
+      export EMACSLOADPATH="$1/share/emacs/site-lisp:${EMACSLOADPATH-}"
   fi
 }