fix: use $USERNAME guaranteed to always be defined in zsh

Fixes #9701

themes/agnoster.zsh-theme

@@ -88,7 +88,7 @@ prompt_end() {
 
 # Context: user@hostname (who am I and where am I)
 prompt_context() {
-  if [[ "$USER" != "$DEFAULT_USER" || -n "$SSH_CLIENT" ]]; then
+  if [[ "$USERNAME" != "$DEFAULT_USER" || -n "$SSH_CLIENT" ]]; then
     prompt_segment black default "%(!.%{%F{yellow}%}.)%n@%m"
   fi
 }