nixVersions.nix_2_28: init at 2.28.1pre

This includes an important fix that's not in a release yet, hence the
"pre" version.

Addendum from Ericson2314: We've discussed for 25.05 going with the
"monolithic build" for the main `nix_2_28`, but we want to land this
right away to unblock preparing PRs to bump versions downstream.

pkgs/tools/package-management/nix/default.nix

@@ -197,6 +197,20 @@ lib.makeExtensible (
 
       nix_2_27 = addTests "nix_2_27" self.nixComponents_2_27.nix-everything;
 
+      nixComponents_2_28 = nixDependencies.callPackage ./modular/packages.nix {
+        version = "2.28.1pre";
+        inherit (self.nix_2_24.meta) maintainers;
+        otherSplices = generateSplicesForNixComponents "nixComponents_2_28";
+        src = fetchFromGitHub {
+          owner = "NixOS";
+          repo = "nix";
+          rev = "9cdf72beaa77f1e6c0faed44872b83783051f20d";
+          hash = "sha256-0sk7cGOdfUA6/AamSsuURHdILxSyw+s2zl7CDaSsawo=";
+        };
+      };
+
+      nix_2_28 = addTests "nix_2_28" self.nixComponents_2_28.nix-everything;
+
       latest = self.nix_2_26;
 
       # The minimum Nix version supported by Nixpkgs

pkgs/tools/package-management/nix/modular/src/libmain/package.nix

@@ -6,6 +6,7 @@
 
   nix-util,
   nix-store,
+  nix-expr,
 
   # Configuration Options
 
@@ -18,7 +19,11 @@ mkMesonLibrary (finalAttrs: {
 
   workDir = ./.;
 
-  propagatedBuildInputs = [
+  propagatedBuildInputs =
+    lib.optionals (lib.versionAtLeast version "2.28") [
+      nix-expr
+    ]
+    ++ [
       nix-util
       nix-store
       openssl