make nix.settings.system-features default mergeable again (NixOS#383052)

Author: Mic92

nixos/modules/config/nix.nix

@@ -45,6 +45,19 @@ let
 
   isNixAtLeast = versionAtLeast (getVersion nixPackage);
 
+  defaultSystemFeatures = [
+    "nixos-test"
+    "benchmark"
+    "big-parallel"
+    "kvm"
+  ] ++ optionals (pkgs.stdenv.hostPlatform ? gcc.arch) (
+      # a builder can run code for `gcc.arch` and inferior architectures
+      [ "gccarch-${pkgs.stdenv.hostPlatform.gcc.arch}" ]
+      ++ map (x: "gccarch-${x}") (
+        systems.architectures.inferiors.${pkgs.stdenv.hostPlatform.gcc.arch} or [ ]
+      )
+    );
+
   legacyConfMappings = {
     useSandbox = "sandbox";
     buildCores = "cores";
@@ -315,20 +328,9 @@ in
 
             system-features = mkOption {
               type = types.listOf types.str;
-              default =
-                [
-                  "nixos-test"
-                  "benchmark"
-                  "big-parallel"
-                  "kvm"
-                ]
-                ++ optionals (pkgs.stdenv.hostPlatform ? gcc.arch) (
-                  # a builder can run code for `gcc.arch` and inferior architectures
-                  [ "gccarch-${pkgs.stdenv.hostPlatform.gcc.arch}" ]
-                  ++ map (x: "gccarch-${x}") (
-                    systems.architectures.inferiors.${pkgs.stdenv.hostPlatform.gcc.arch} or [ ]
-                  )
-                );
+              # We expose system-featuers here and in config below.
+              # This allows users to access the default value via `options.nix.settings.system-features`
+              default = defaultSystemFeatures;
               defaultText = literalExpression ''[ "nixos-test" "benchmark" "big-parallel" "kvm" "gccarch-<arch>" ]'';
               description = ''
                 The set of features supported by the machine. Derivations
@@ -385,6 +387,7 @@ in
       trusted-public-keys = [ "cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY=" ];
       trusted-users = [ "root" ];
       substituters = mkAfter [ "https://cache.nixos.org/" ];
+      system-features = defaultSystemFeatures;
     };
   };
 }