nixos/anbox: use --daemon to squash warning

As noted in https://github.com/NixOS/nixpkgs/pull/102341 this is not
actually running as a forked process. It only tells the process that it
is running "as a daemon, so shut the warning up".

See `daemon_` here

 - 9de4e87cdd/src/anbox/cmds/container_manager.cpp (L38-L79)

It is **strictly** used to hide that message.

Co-authored-by: Matt Votava <mvnetbiz@gmail.com>

nixos/modules/virtualisation/anbox.nix

@@ -166,7 +166,8 @@ in
             --container-network-gateway=${cfg.ipv4.gateway.address} \
             --container-network-dns-servers=${cfg.ipv4.dns} \
             --use-rootfs-overlay \
-            --privileged
+            --privileged \
+            --daemon
         '';
       };
     };