zk: Set working dir to wiki for aliases

Instead of finding the notebook at the wikiroot, we directly change the
current working dir to the notebook for the runtime of the zk command.

This has a couple advantages: Any other commands executed will also pick
up the correct directory, and creating notes in a specific directory
also just _works_ by giving a directory in the notes dir even if we are
somewhere else.

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

@@ -2,7 +2,7 @@
 #
 
 _zk_wiki() {
-    zk --notebook-dir="$WIKIROOT" "$@"
+    zk --working-dir="$WIKIROOT" "$@"
 }
 
 n() {