[backend] publish appx files

they are signed internal, so no additional sha256 or signatures are
needed

src/backend/bs_publish

@@ -1856,6 +1856,8 @@ sub publish {
 	$p = "$1/$bin";
       } elsif ($bin =~ /\.(?:AppImage|AppImage.zsync|snap|exe)?$/) {
 	$p = "$bin";
+      } elsif ($bin =~ /\.appx$/) {
+	$p = "$bin";
       } elsif ($bin =~ /\.d?rpm$/) {
 	# legacy format
 	my $q = Build::query("$r/$rbin", 'evra' => 1);