fix(ops/nixos): don't set nixpkgs.pkgs explicitly

As soon as you pass in an already-instantiated nixpkgs version, it will
cause nixpkgs.hostPlatform etc. to be not applied.

This means it's impossible to describe the architecture of a VM closure
you're deploying, and have it deviate from the machine you're evaluating
from, making it quite hard to deploy that x86_64-linux machine from
aarch64-linux (where I'm writing this commit message from).

Drop explicitly passing in nixpkgs.path, and set nixpkgs.hostPlatform
explicitly for all remaining system configurations in the repository
where not already set.

Change-Id: Ie2a596e0826da54674b4f02fcd8fed3569fee0a4
Reviewed-on: https://cl.snix.dev/c/snix/+/30104
Autosubmit: Florian Klink <flokli@flokli.de>
Tested-by: besadii
Reviewed-by: Ryan Lahfa <masterancpp@gmail.com>

ops/nixos.nix

@@ -1,12 +1,10 @@
 # Helper functions for instantiating depot-compatible NixOS machines.
 { depot, lib, pkgs, ... }@args:
 
-let inherit (lib) findFirst isAttrs;
+let inherit (lib) findFirst;
 in rec {
   # This provides our standard set of arguments to all NixOS modules.
   baseModule = { ... }: {
-    # Ensure that pkgs == third_party.nix
-    nixpkgs.pkgs = depot.third_party.nixpkgs;
     nix.nixPath =
       let
         # Due to nixpkgsBisectPath, pkgs.path is not always in the nix store