cups: mount private /tmp

printer driver and wrapper are often not written with security in mind.

While reviewing https://github.com/NixOS/nixpkgs/pull/25654 I found
a symlink-race vulnerability within the wrapper code, when writing
unique files in /tmp.
I expect this script to be reused in other models as well
as similar vulnerabilities in the code of other vendors. Therefore
I propose to make /tmp of cups.service private so that only processes
with the same privileges are able to access these files.

nixos/modules/services/printing/cupsd.nix

@@ -324,6 +324,8 @@ in
               fi
             ''}
           '';
+
+          serviceConfig.PrivateTmp = true;
       };
 
     systemd.services.cups-browsed = mkIf avahiEnabled