Change all whoami calls for $USER variable

This avoid spawning additional processes, as the $USER variable will
**always** contain the current logged-in user.

themes/flazz.zsh-theme

@@ -1,4 +1,4 @@
-if [ "$(whoami)" = "root" ]
+if [ "$USER" = "root" ]
 then CARETCOLOR="red"
 else CARETCOLOR="blue"
 fi