flatpak: Use a set() instead of array when gathering locale variants.

Minor optimization -

This prevents having duplicates of locale names - if the locale is
'de', you'd end up with  the initial 'de', then the single variant
added would also be 'de'.

usr/lib/python3/dist-packages/mintcommon/installer/_flatpak.py

@@ -189,9 +189,9 @@ def update_xmlb_info(self):
             return
 
         locale = self.as_pool.get_locale()
-        locale_variants = [locale]
+        locale_variants = {locale}
         for v in GLib.get_locale_variants(locale):
-            locale_variants.append(locale_to_bcp47(v))
+            locale_variants.add(locale_to_bcp47(v))
 
         for comp_id in self.app_dict.keys():
             try: