fix(views/kit): allow setting localSystem when using tvl-kit

Because the passing around of externalArgs only happened in the readTree
instantiation in the repo root default.nix, but not in views/kit/
default.nix, it was not possible to get tvl-kit to instantiate the
bundled nixpkgs with a custom system.

This fixes invocations like

```
tvl-kit = import (sources.tvl-kit) { localSystem = "aarch64-linux"; };
```

Change-Id: I3a633e4d695d266459400ba74fc0693ecc5bfb54
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8197
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

views/kit/default.nix

@@ -9,9 +9,10 @@
 { pkgs ? (import ./nixpkgs {
     depotOverlays = false;
     depot.third_party.sources = import ./sources { };
+    externalArgs = args;
   })
 , ...
-}:
+}@args:
 
 pkgs.lib.fix (self: {
   besadii = import ./besadii {