nixpkgs: add an update script

this only updates the master branch -- for now

pkgs/additional/nixpkgs/default.nix

@@ -1,5 +1,6 @@
 # XXX: this is in the bootstrap path;
-# i'm pretty much restricted to using just `builtin`s here.
+# this means it has to be evaluatable using only builtins,
+# though i'm free to include optional functionality (e.g. update scripts) so long as i gate it behind availability checks.
 #
 # branch workflow:
 # - daily:
@@ -20,7 +21,9 @@
 , doPatch ? true
 , localSystem ? builtins.currentSystem  #< not available in pure mode
 , system ? localSystem
+#VVV these may or may not be available when called VVV
 , fetchzip ? builtins.fetchTarball
+, nix-update-script ? null
 }:
 let
   lock = {
@@ -31,9 +34,8 @@ let
     staging-next.rev = "5aa86ae5585cd46299ee46682fda8a9b76baf2ae";
     staging-next.sha256 = "sha256:1dqws7fmdv4l9mk45i88ivzlxzm2b4lbdg9rg4am0z1alsz36dzz";
   };
-  unpatchedSrc = let
-    lock' = lock."${variant}";
-  in fetchzip {
+  lock' = lock."${variant}";
+  unpatchedSrc = fetchzip {
     url = "https://github.com/NixOS/nixpkgs/archive/${lock'.rev}.tar.gz";
     inherit (lock') sha256;
   };
@@ -66,4 +68,15 @@ let
 in
   # N.B.: this is crafted to allow `nixpkgs.FOO` from other nix code
   # AND `nix-build -A nixpkgs`
-  (import "${src}" args) // src
+  (import "${src}" args) // src // {
+    # attributes needed for update scripts
+    name = "nixpkgs";
+    version = "24.05-unstable-2024-06-xx";
+    src = unpatchedSrc // {
+      inherit (lock') rev;
+    };
+  } // (if nix-update-script != null then {
+    passthru.updateScript = nix-update-script {
+      extraArgs = [ "--version" "branch" ];
+    };
+  } else {})