programs: tuba: sandbox with bwrap

it complains "Fontconfig error: No writable cache directories"
seeeeeveral times. not sure if that's new or not. no obvious
consequences.

hosts/common/programs/tuba.nix

@@ -1,6 +1,7 @@
 { ... }:
 {
   sane.programs.tuba = {
+    sandbox.method = "bwrap";
     suggestedPrograms = [ "gnome-keyring" ];
   };
 }