Fix open_command nohup call

There a syntax error if $cmd_var contains more than one word,
so we drop the current way to prepend nohup and use a simpler
if-else form.

lib/functions.zsh

@@ -29,9 +29,11 @@ function open_command() {
   esac
 
   # don't use nohup on OSX
-  [[ "$OSTYPE" != darwin* ]] && open_cmd="nohup $open_cmd"
-
+  if [[ "$OSTYPE" == darwin* ]]; then
     $open_cmd "$@" &>/dev/null
+  else
+    nohup $open_cmd "$@" &>/dev/null
+  fi
 }
 
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