zk: Check WIKIROOT before adding n alias for zk

Since this function also requires a WIKIROOT to be available in the
environment, we should only define it if that variable is not empty --
just like all the other zk aliases and functions.

writing/zk/config/sh/alias.d/zk.sh

@@ -1,5 +1,7 @@
 #!/usr/bin/env sh
-#
+
+# We have a local wiki
+if [ -n "${WIKIROOT}" ]; then
 
     _zk_wiki() {
         zk --working-dir="$WIKIROOT" "$@"
@@ -13,8 +15,6 @@ n() {
         fi
     }
 
-# We have a local wiki
-if [ -n "${WIKIROOT}" ]; then
     # open notes with my vim zettelkasten plugin
     # TODO better implementation conditional on zk.nvim & zettelkasten existing
     # nvim +'lua pcall(require "zk.commands"') --headless +qa 2>&1 or similar to check - but slow