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/build01/default.nix

@@ -12,6 +12,8 @@ in
     (depot.third_party.agenix.src + "/modules/age.nix")
   ];
 
+  nixpkgs.hostPlatform = "x86_64-linux";
+
   # Machine model taken from project Floral.
   boot.isContainer = true;