builders/netboot: add separate firmware_part output

services/baremetal-builder/netboot.nix

@@ -75,6 +75,7 @@ in
             system.build.usbImage = pkgs.callPackage ({ stdenv, runCommand, dosfstools, e2fsprogs, mtools, libfaketime, util-linux, nukeReferences }:
               runCommand "boot-img-${node.config.networking.hostName}" {
                 nativeBuildInputs = [ dosfstools e2fsprogs libfaketime mtools util-linux ];
+                outputs = [ "out" "firmware_part" ];
               } ''
                 export img=$out
                 truncate -s 40M $img
@@ -123,6 +124,8 @@ in
                 # Verify the FAT partition before copying it.
                 fsck.vfat -vn firmware_part.img
                 dd conv=notrunc if=firmware_part.img of=$img seek=$START count=$SECTORS
+
+                cp firmware_part.img $firmware_part
               ''
             ) {};
           }