nixos/testing: Drop darwin from default hydraPlatforms

Litmus test: the following attribute disappears

nix eval -f pkgs/top-level/release.nix tests.testers.nixosTest-example.x86_64-darwin.outPath

Changed files
+5 -3
nixos
lib
testing
+5 -3
nixos/lib/testing/meta.nix
···
{ lib, ... }:
let
-
inherit (lib) types mkOption literalExpression;
+
inherit (lib) types mkOption literalMD;
in
{
options = {
···
};
hydraPlatforms = mkOption {
type = types.listOf types.raw;
-
default = config.platforms;
-
defaultText = literalExpression "meta.platforms";
+
# Ideally this would default to `platforms` again:
+
# default = config.platforms;
+
default = lib.platforms.linux;
+
defaultText = literalMD "`lib.platforms.linux` only, as the `hydra.nixos.org` build farm does not currently support virtualisation on Darwin.";
description = ''
Sets the [`meta.hydraPlatforms`](https://nixos.org/manual/nixpkgs/stable/#var-meta-hydraPlatforms) attribute on the [{option}`test`](#test-opt-test) derivation.
'';