amazon-image.nix: Resolve failure to include resize2fs

Since 34234dcb51, for resize2fs to be automatically included in
initrd, a filesystem needed for boot must be explicitly defined as an
ext* type filesystem.

nixos/maintainers/scripts/ec2/amazon-image.nix

@@ -53,6 +53,7 @@ in {
     pkgs = import ../../../.. { inherit (pkgs) system; }; # ensure we use the regular qemu-kvm package
     partitionTableType = if config.ec2.hvm then "legacy" else "none";
     diskSize = cfg.sizeMB;
+    fsType = "ext4";
     configFile = pkgs.writeText "configuration.nix"
       ''
         {

nixos/modules/virtualisation/amazon-image.nix

@@ -31,6 +31,7 @@ in
 
     fileSystems."/" = {
       device = "/dev/disk/by-label/nixos";
+      fsType = "ext4";
       autoResize = true;
     };