buildGoModule: remove compatibility layer for directly specified CGO_ENABLED

doc/release-notes/rl-2511.section.md

···
 
 - `python3Packages.bjoern` has been removed, as the upstream is unmaintained and it depends on a 14-year-old version of http-parser with numerous vulnerabilities.
 
+- `buildGoModule` removes the compatibility layer of `CGO_ENABLED` not specified via `env`.
+  Specifying `CGO_ENABLED` directly now results in an error.
+
 - `buildGoModule` now warns if `<pkg>.passthru.overrideModAttrs` is lost during the overriding of its result packages.
 
 - `gentium` package now provides `Gentium-*.ttf` files, and not `GentiumPlus-*.ttf` files like before. The font identifiers `Gentium Plus*` are available in the `gentium-plus` package, and if you want to use the more recently updated package `gentium` [by sil](https://software.sil.org/gentium/), you should update your configuration files to use the `Gentium` font identifier.

pkgs/build-support/go/module.nix

···
   constructDrv = stdenv.mkDerivation;
   excludeDrvArgNames = [
     "overrideModAttrs"
-    # Compatibility layer to the directly-specified CGO_ENABLED.
-    # TODO(@ShamrockLee): Remove after Nixpkgs 25.05 branch-off
-    "CGO_ENABLED"
   ];
   extendDrvArgs =
     finalAttrs:
···
         GO111MODULE = "on";
         GOTOOLCHAIN = "local";
 
-        CGO_ENABLED =
-          args.env.CGO_ENABLED or (
-            if args ? CGO_ENABLED then
-              # Compatibility layer to the CGO_ENABLED attribute not specified as env.CGO_ENABLED
-              # TODO(@ShamrockLee): Remove and convert to
-              # CGO_ENABLED = args.env.CGO_ENABLED or go.CGO_ENABLED
-              # after the Nixpkgs 25.05 branch-off.
-              lib.warn
-                "${finalAttrs.finalPackage.meta.position}: buildGoModule: specify CGO_ENABLED with env.CGO_ENABLED instead."
-                args.CGO_ENABLED
-            else
-              go.CGO_ENABLED
-          );
+        CGO_ENABLED = args.env.CGO_ENABLED or go.CGO_ENABLED;
       };
 
       GOFLAGS =