nixos/systemd-boot: use new combined memtest86plus binary

memtest86plus puts the path to the binary at a passthru attr

after memtest86plus 8 there's a single combined EFI+legacy file

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

@@ -391,7 +391,7 @@ in
       type = types.attrsOf types.path;
       default = { };
       example = literalExpression ''
-        { "efi/memtest86/memtest.efi" = "''${pkgs.memtest86plus}/memtest.efi"; }
+        { "efi/memtest86/memtest.efi" = pkgs.memtest86plus.efi; }
       '';
       description = ''
         A set of files to be copied to {file}`$BOOT`.
@@ -578,7 +578,7 @@ in
 
     boot.loader.systemd-boot.extraFiles = mkMerge [
       (mkIf cfg.memtest86.enable {
-        "efi/memtest86/memtest.efi" = "${pkgs.memtest86plus.efi}";
+        "efi/memtest86/memtest.efi" = pkgs.memtest86plus.efi;
       })
       (mkIf cfg.netbootxyz.enable {
         "efi/netbootxyz/netboot.xyz.efi" = "${pkgs.netbootxyz-efi}";