feat(configuration.nix): add nix-community cachix substituter

[phorcys420]

What

Add nix-community.cachix.org as an extra binary cache in the shared nix.settings.

nix.settings.extra-substituters = [ "https://nix-community.cachix.org" ];
nix.settings.extra-trusted-public-keys = [
  "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs="
];

Why

Pull community-built closures (disko and other nix-community derivations) from a
binary cache instead of building them locally. extra- appends to the defaults,
so cache.nixos.org stays in place. Applies to every host via configuration.nix.

Note

nixpkgs is pinned to the released nixos-25.11 channel, so podman is already
served by cache.nixos.org and should not build from source. This cache mainly
helps with disko and other community derivations. If podman is genuinely
rebuilding, the cause is likely elsewhere (an overlay touching its deps, an
arch/unfree gap, or a stale substituter) and worth a separate look.

---

Drafted by Coder Agents on behalf of @phorcys420.