modules/nixos/common/update: use grub.extraInstallCommands for boot diff

modules/nixos/common/update.bash

@@ -12,11 +12,6 @@ nix-env --profile /nix/var/nix/profiles/system --set "$p"
 booted="$(readlink /run/booted-system/{initrd,kernel,kernel-modules} && cat /run/booted-system/kernel-params)"
 built="$(readlink "$p"/{initrd,kernel,kernel-modules} && cat "$p"/kernel-params)"
 if [[ $booted != "$built" ]]; then
-  if [[ -e /run/current-system ]]; then
-    echo "--- diff to current-system"
-    nvd diff /run/current-system "$p"
-    echo "---"
-  fi
   /nix/var/nix/profiles/system/bin/switch-to-configuration boot
   # don't use kexec if system is virtualized, reboots are fast enough
   if ! systemd-detect-virt -q; then

modules/nixos/common/update.nix

@@ -21,6 +21,16 @@
     script = builtins.readFile ./update.bash;
   };
 
+  # https://gist.github.com/Ma27/6650d10f772511931647d3189b3eb1d7
+  # https://github.com/NuschtOS/nixos-modules/blob/39d26dddae2f117d7f9c33dd1efc866ff684ff94/modules/nix.nix
+  boot.loader.grub.extraInstallCommands = ''
+    if [[ "''${NIXOS_ACTION-}" == boot && -e /run/current-system && -e "''${1-}" ]]; then
+      echo "--- diff to current-system"
+      ${pkgs.nvd}/bin/nvd --nix-bin-dir=${config.nix.package}/bin diff /run/current-system "''${1-}"
+      echo "---"
+    fi
+  '';
+
   systemd.timers.update-host = {
     wantedBy = [ "timers.target" ];
     timerConfig.OnBootSec = "5m";