fix(3p/nixpkgs): pass localSystem from depot to nixpkgs

Change-Id: I75d2ad14ebc81a76cfa7c8d83e77b4a124b03466
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5846
Autosubmit: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

third_party/nixpkgs/default.nix

@@ -8,7 +8,12 @@
 # in //default.nix passes this attribute as the `pkgs` argument to all
 # readTree derivations.
 
-{ depot ? { }, externalArgs ? { }, depotOverlays ? true, ... }:
+{ depot ? { }
+, externalArgs ? { }
+, depotOverlays ? true
+, localSystem ? builtins.currentSystem
+, ...
+}:
 
 let
   # import the nixos-unstable package set, or optionally use the
@@ -48,6 +53,8 @@ import nixpkgsSrc {
       allowBroken = true;
     };
 
+  inherit localSystem;
+
   overlays = [
     commitsOverlay
     stableOverlay