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/machines/meta01/default.nix

@@ -25,6 +25,8 @@ in
     (depot.third_party.disko.src + "/module.nix")
   ];
 
+  nixpkgs.hostPlatform = "x86_64-linux";
+
   infra.hardware.hetzner-cloud = {
     enable = true;
     ipv6 = "2a01:4f8:c013:4a58::1/64";