install: Use ONE_DIR flag

Since you can't really install in more than
one location at a time.

Closes: #2534
Approved by: alexlarsson

app/flatpak-builtins-install.c

@@ -276,7 +276,7 @@ flatpak_builtin_install (int argc, char **argv, GCancellable *cancellable, GErro
   g_option_context_set_translation_domain (context, GETTEXT_PACKAGE);
 
   if (!flatpak_option_context_parse (context, options, &argc, &argv,
-                                     FLATPAK_BUILTIN_FLAG_STANDARD_DIRS,
+                                     FLATPAK_BUILTIN_FLAG_ONE_DIR,
                                      &dirs, cancellable, error))
     return FALSE;