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/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