Rename gummiboot module

nixos/modules/module-list.nix

···
   ./system/boot/loader/grub/grub.nix
   ./system/boot/loader/grub/ipxe.nix
   ./system/boot/loader/grub/memtest.nix
-  ./system/boot/loader/gummiboot/gummiboot.nix
   ./system/boot/loader/init-script/init-script.nix
   ./system/boot/loader/loader.nix
   ./system/boot/loader/raspberrypi/raspberrypi.nix
+  ./system/boot/loader/systemd-boot/systemd-boot.nix
   ./system/boot/luksroot.nix
   ./system/boot/modprobe.nix
   ./system/boot/networkd.nix

nixos/modules/system/boot/loader/gummiboot/gummiboot.nix nixos/modules/system/boot/loader/systemd-boot/systemd-boot.nix

···
   efi = config.boot.loader.efi;
 
   gummibootBuilder = pkgs.substituteAll {
-    src = ./gummiboot-builder.py;
+    src = ./systemd-boot-builder.py;
 
     isExecutable = true;