modules/boot: Specify the type for tmpfs mounts

https://github.com/NixOS/nixpkgs/pull/107497 broke booting on many systems that
use tmpOnTmpfs due to the lack of specifying the mount type.

This commit explicitly adds the mount type, which should fix booting
such systems.

The original change may want to be revisited however too.

nixos/modules/system/boot/tmp.nix

@@ -34,6 +34,7 @@ with lib;
       {
         what = "tmpfs";
         where = "/tmp";
+        type = "tmpfs";
         mountConfig.Options = [ "mode=1777" "strictatime" "rw" "nosuid" "nodev" "size=50%" ];
       }
     ];