containers.*.config: reuse host `nixpkgs.pkgs` if defined

The minimum reproduction for the problem I'm trying to solve is that
the following NixOS test with a trivial NixOS container:

```
{ inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/24.05";

    flake-utils.url = "github:numtide/flake-utils/v1.0.0";
  };

  outputs = { flake-utils, nixpkgs, self, ... }:
    flake-utils.lib.eachDefaultSystem (system: {
      checks.default = nixpkgs.legacyPackages."${system}".nixosTest {
        name = "test";

        nodes.machine.containers.tutorial.config = { };

        testScript = "";
      };
    });
}
```

… fails with the following error message:

```
error: Neither nodes.machine.nixpkgs.hostPlatform nor the legacy option nodes.machine.nixpkgs.system has been set.
You can set nodes.machine.nixpkgs.hostPlatform in hardware-configuration.nix by re-running
a recent version of nixos-generate-config.
The option nodes.machine.nixpkgs.system is still fully supported for NixOS 22.05 interoperability,
but will be deprecated in the future, so we recommend to set nodes.machine.nixpkgs.hostPlatform.
```

The root of the problem appears to be that in
`nixos/modules/virtualisation/nixos-containers.nix` there is support
for deriving the guest's `nixpkgs.hostPlatform` or
`nixpkgs.localSystem` from the corresponding host's values, but this
doesn't work if the host sets `nixpkgs.pkgs` instead of one of those
values.  In fact, this is what happens when using `pkgs.nixosTest`
(which sets `nixpkgs.pkgs` in
`pkgs/build-support/testers/default.nix`).

The solution I went with was to forward the `nixpkgs.pkgs` setting from
the host to the guest, but only if it is defined (matching the same
treatment as `nixpkgs.hostPlatform` and `nixpkgs.localSystem`.

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