feat(3p/nixpkgs): Plumb through externalArgs.nixpkgsBisectPath

This lets the import of the depot root accept an additional argument
called `externalArgs`, which can be used to pass additional arguments
into a depot package set.

This is used in //third_party/nixpkgs for replacing the source of the
nixos-unstable channel with a path. With this we can bisect the
nixpkgs used in third_party easily.

Change-Id: I4f65eb3d6b521ed9f437649b7b068f1e6ab8210f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2925
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

default.nix

@@ -5,7 +5,7 @@
 # This makes packages accessible via the Nixery instance that is configured to
 # use this repository as its nixpkgs source.
 
-{ ... }@args:
+{ nixpkgsBisectPath ? null, ... }@args:
 
 with builtins;
 
@@ -24,6 +24,13 @@ let
     # Pass third_party as 'pkgs' (for compatibility with external
     # imports for certain subdirectories)
     pkgs = depot.third_party.nixpkgs;
+
+    # Pass arguments passed to the entire depot through, for packages
+    # that would like to add functionality based on this.
+    #
+    # Note that it is intended for exceptional circumstance, such as
+    # debugging by bisecting nixpkgs.
+    externalArgs = args;
   };
 
   readTree' = import ./nix/readTree {};