refactor: use environment variable for zettelkasten path

dots/.bin/zk

@@ -1,7 +1,9 @@
 #!/usr/bin/env bash
 
+current_zettel_path="$ZK_PATH/$(cat "$ZK_PATH/current-zettel.txt")"
+
 if [ "$TERM_PROGRAM" = tmux ]; then
-  cd ~/.zk && $EDITOR "$(cat ~/.zk/current-zettel.txt)"
+  cd "$ZK_PATH" && $EDITOR "$current_zettel_path"
 else
   echo 'Not in tmux'
   echo 'Choose an option:'
@@ -18,12 +20,12 @@ else
       else
         # Create session with a window named 'zk' and start nvim
         tmux new-session -s zk -n zk -d
-        tmux send-keys -t zk:zk "cd ~/.zk && $EDITOR \"\$(cat ~/.zk/current-zettel.txt)\"" Enter
+        tmux send-keys -t zk:zk "cd $ZK_PATH && $EDITOR $current_zettel_path" Enter
         tmux attach -t zk
       fi
       ;;
     2)
-      cd ~/.zk && $EDITOR "$(cat ~/.zk/current-zettel.txt)"
+      cd "$ZK_PATH" && $EDITOR "$current_zettel_path"
       ;;
     *)
       echo 'Not opening Zettelkasten'