nixos/testing/nodes: Do allow aliases

Aliases exist for a reason. Sure it is nice to make sure that
some aliases aren't used within Nixpkgs, but this creates two problems
which are far worse than your failing to meet your neatness compulsions.

- Users encounter missing attributes, https://github.com/NixOS/nixpkgs/issues/264577
  wasting their time, stalling their progress, and even occupying others
  time that would be better spent on fixing *real* issues.

- Hydra doesn't treat evaluation errors seriously enough, with the
  effect that actual relevant test failures are masked by evaluation
  failures such as those caused by this no aliases business.

- We don't even have the infrastructure to get rid of aliases, because
  all warnings in package attributes are disallowed by Nixpkgs CI
  tooling, last I checked.

Before re-disabling this, make sure that

- An actually helpful deprecation process is in place.

- Aliases are still allowed when `nixos-lib.runTests` and
  `pkgs.testers.runNixOSTest` are invoked by external projects.
  For instance, `all-tests.nix` could provide such an
  override (e.g. with `newScope`).

nixos/lib/testing/nodes.nix

@@ -32,9 +32,6 @@ let
             key = "nodes.nix-pkgs";
             config = optionalAttrs (!config.node.pkgsReadOnly) (
               mkIf (!options.nixpkgs.pkgs.isDefined) {
-                # Ensure we do not use aliases. Ideally this is only set
-                # when the test framework is used by Nixpkgs NixOS tests.
-                nixpkgs.config.allowAliases = false;
                 # TODO: switch to nixpkgs.hostPlatform and make sure containers-imperative test still evaluates.
                 nixpkgs.system = hostPkgs.stdenv.hostPlatform.system;
               }