For Sommelier (ie: ChromeOS), use CLIPBOARD not PRIMARY

ChromeOS (as of Jun 2023) effectivly makes PRIMARY useless.  Just
do the wrong thing and use CLIPBOARD instead.  This makes it much
more likely that SyncTERM selections will clobber CLIPBOARD
unintentionally, but we have no choice... DigitalMan doesn't use
X11 mode solely because ChromeOS sucks at X11.