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/ys.zsh-theme

@@ -33,7 +33,7 @@ ${git_info} \
 %{$fg[white]%}[%*]
 %{$terminfo[bold]$fg[red]%}$ %{$reset_color%}"
 
-if [[ "$(whoami)" == "root" ]]; then
+if [[ "$USER" == "root" ]]; then
 PROMPT="
 %{$terminfo[bold]$fg[blue]%}#%{$reset_color%} \
 %{$bg[yellow]%}%{$fg[cyan]%}%n%{$reset_color%} \