swap: don't create filesystem if randomEncryption is enabled

This is not an issue when initially creating the partition because
device mapper will just map on top of the filesystem, but it breaks
incremental updates because the check for `TYPE=` will fail, and the
subsequent attempt to run `mkswap` also fails:

mkswap: cannot open /dev/disk/by-partlabel/disk-nix-store-swap: Device or resource busy

Instead, just don't do anything, NixOS will take care of it.

lib/types/swap.nix

@@ -60,7 +60,8 @@
     };
     _create = diskoLib.mkCreateOption {
       inherit config options;
-      default = ''
+      # TODO: we don't support encrypted swap yet
+      default = lib.optionalString (!config.randomEncryption) ''
         if ! blkid "${config.device}" -o export | grep -q '^TYPE='; then
           mkswap \
             ${toString config.extraArgs} \