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

Fixes #9701

plugins/screen/screen.plugin.zsh

@@ -2,7 +2,7 @@
 # of the tab window should be.
 if [[ "$TERM" == screen* ]]; then
   if [[ $_GET_PATH == '' ]]; then
-    _GET_PATH='echo $PWD | sed "s/^\/Users\//~/;s/^\/home\//~/;s/^~$USER/~/"'
+    _GET_PATH='echo $PWD | sed "s/^\/Users\//~/;s/^\/home\//~/;s/^~$USERNAME/~/"'
   fi
   if [[ $_GET_HOST == '' ]]; then
     _GET_HOST='echo $HOST | sed "s/\..*//"'
@@ -51,4 +51,4 @@ if [[ "$TERM" == screen* ]]; then
     eval "tab_hardstatus=$TAB_HARDSTATUS_PREFIX:$TAB_HARDSTATUS_PROMPT"
     screen_set $tab_title $tab_hardstatus
   }
-fi
+fi