fix(tvix): Pass all args when auto-calling a function with an ellipsis

The command line options --arg and --argstr that are used by a bunch of
CLI commands to pass arguments to top-level functions in files go
through the same code-path as auto-calling top-level functions with
their default arguments - this, however, was only passing the arguments
that were *explicitly* mentioned in the formals of the function - in the
case of an as-pattern with an ellipsis (eg args @ { ... }) extra passed
arguments would get omitted. This fixes that to instead pass *all*
specified auto args in the case that our function has an ellipsis.

Submitted upstream at https://github.com/NixOS/nix/pull/3965

Fixes: #46
Change-Id: I32b7ee0e5bacf75b2bc43a3f0796f533f4bd5959
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1863
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI

third_party/nix/src/nix-build/nix-build.cc

@@ -270,6 +270,7 @@ static void _main(int argc, char** argv) {
 
   if (packages) {
     std::ostringstream joined;
+    // TODO(grfn): Generate a syntax tree here, not a string
     joined << "with import <nixpkgs> { }; (pkgs.runCommandCC or "
               "pkgs.runCommand) \"shell\" { buildInputs = [ ";
     for (const auto& i : left) {