Select swap partition format automatically

lib/modules/partition.nix

@@ -192,6 +192,10 @@
           format = mkOption {
             type = types.enum (builtins.attrValues fs);
             description = "The file system format of the partition.";
+            default =
+              if (isSwap config)
+              then fs.swap
+              else throw("Partition format not specified.");
           };
 
           size = mkOption {
@@ -254,7 +258,6 @@
             Swap = {
               index = 2;
               type = "swap";
-              format = fs.ntfs;
             };
 
             OS = {