tests/evalNixpkgs: Split out into its own file and set NIX_CONFIG

This adds a way to control the deprecated features that need to be
enable while evaluating the specific nixpkgs version

Change-Id: Ia2b9a29f1966d4d5f32239a36ba8270330007da9

flake.nix

@@ -418,15 +418,9 @@
 
             # Make sure that nix-env still produces the exact same result
             # on a particular version of Nixpkgs.
-            evalNixpkgs =
-              with nixpkgsFor.x86_64-linux.native;
-              runCommand "eval-nixos" { buildInputs = [ nix ]; } ''
-                type -p nix-env
-                # Note: we're filtering out nixos-install-tools because https://github.com/NixOS/nixpkgs/pull/153594#issuecomment-1020530593.
-                time nix-env --store dummy:// -f ${nixpkgs-regression} -qaP --drv-path | sort | grep -v nixos-install-tools > packages
-                [[ $(sha1sum < packages | cut -c1-40) = 402242fca90874112b34718b8199d844e8b03d12 ]]
-                mkdir $out
-              '';
+            evalNixpkgs = nixpkgsFor.x86_64-linux.native.callPackage ./tests/nixpkgs/eval.nix {
+              inherit nixpkgs-regression;
+            };
 
             nixpkgsLibTests = forAllSystems (
               system:

tests/nixpkgs/eval.nix

@@ -0,0 +1,26 @@
+{
+  lib,
+  runCommand,
+  nix,
+
+  nixpkgs-regression,
+}:
+
+let
+  inherit (lib) concatStringsSep;
+
+  deprecatedFeatures = [ ];
+in
+
+runCommand "eval-nixos"
+  {
+    buildInputs = [ nix ];
+    env.NIX_CONFIG = "extra-deprecated-features = ${concatStringsSep " " deprecatedFeatures}";
+  }
+  ''
+    type -p nix-env
+    # Note: we're filtering out nixos-install-tools because https://github.com/NixOS/nixpkgs/pull/153594#issuecomment-1020530593.
+    time nix-env --store dummy:// -f ${nixpkgs-regression} -qaP --drv-path | sort | grep -v nixos-install-tools > packages
+    [[ $(sha1sum < packages | cut -c1-40) = 402242fca90874112b34718b8199d844e8b03d12 ]]
+    mkdir $out
+  ''