New rebuild mechanism

Put most of it as a flake app, so we can mess with it without relying on
`rb` being rebuilt. Also nom nom!

os/default.nix

@@ -9,7 +9,6 @@
     ./common.nix
     ./desktop
     ./dev
-    ./diff
     disko.nixosModules.disko
     ./gaming
     ./geoffrey.nix

os/diff/default.nix

@@ -1,14 +0,0 @@
-{ pkgs, lib, config, ... }:
-{
-  config = {
-    system.activationScripts.diff = {
-      supportsDryActivation = true;
-      text = ''
-        if [ -h /run/current-system ]
-        then
-            ${pkgs.nvd}/bin/nvd --nix-bin-dir=${pkgs.nix}/bin diff /run/current-system "$systemConfig"
-        fi
-      '';
-    };
-  };
-}

os/rebuild.sh

@@ -0,0 +1,54 @@
+# Handle arguments
+self="$1"
+verb="$2"
+shift
+shift
+
+if [ "$verb" != "build" ] && [ "$verb" != "test" ] && [ "$verb" != "boot" ] && [ "$verb" != "switch" ] && [ "$verb" != "confirm" ]
+then
+  echo "Action should be one of: build, test, boot, switch, confirm"
+  exit 2
+fi
+
+# Build, looking nice
+tmpdir="$(mktemp -d)"
+# sudo so the eval cache is shared with nixos-rebuild
+sudo nom build "$self#nixosConfigurations.$HOSTNAME.config.system.build.toplevel" -o "$tmpdir/toplevel" "$@"
+toplevel="$(readlink "$tmpdir/toplevel")"
+rm -rf "$tmpdir"
+
+# Show diff
+nvd diff /nix/var/nix/profiles/system "$toplevel"
+
+# Figure out specialisation
+specialisationArgs=""
+currentSystem="$(readlink /run/current-system)"
+while read -r specialisation
+do
+    if [ "$(readlink "/nix/var/nix/profiles/system/specialisation/$specialisation")" = "$currentSystem" ]
+    then
+        specialisationArgs=("--specialisation" "$specialisation")
+    fi
+done <<< "$(ls /nix/var/nix/profiles/system/specialisation)"
+
+# Apply
+if [ "$verb" = "confirm" ]
+then
+  echo "Apply configuration? [y/N]"
+  read -r confirm
+fi
+if [ "$verb" = "test" ] || [ "$verb" = "switch" ] || [ "$confirm" = "y" ]
+then
+  sudo nixos-rebuild --flake "$self#$HOSTNAME" test "${specialisationArgs[@]}" "$@"
+fi
+
+# Set as boot
+if [ "$verb" = "confirm" ]
+then
+  echo "Set configuration as boot? [y/N]"
+  read -r confirm
+fi
+if [ "$verb" = "boot" ] || [ "$verb" = "switch" ] || [ "$confirm" = "y" ]
+then
+  sudo nixos-rebuild --flake "$self#$HOSTNAME" boot "$@"
+fi