glibc: NIX_CFLAGS_COMPILE -> always string

pkgs/development/libraries/glibc/default.nix

@@ -47,14 +47,14 @@ callPackage ./common.nix { inherit stdenv; } {
         #       musl-specific flags below.
         #       At next change to non-musl glibc builds, remove this `then`
         #       and the above condition, instead keeping only the `else` below.
-        then (if withGd then "-Wno-error=stringop-truncation" else null)
+        then (stdenv.lib.optionalString withGd "-Wno-error=stringop-truncation")
         else
-          builtins.concatLists [
+          builtins.toString (builtins.concatLists [
             (stdenv.lib.optional withGd "-Wno-error=stringop-truncation")
             # Fix -Werror build failure when building glibc with musl with GCC >= 8, see:
             # https://github.com/NixOS/nixpkgs/pull/68244#issuecomment-544307798
             (stdenv.lib.optional stdenv.hostPlatform.isMusl "-Wno-error=attribute-alias")
-          ];
+          ]);
 
     # When building glibc from bootstrap-tools, we need libgcc_s at RPATH for
     # any program we run, because the gcc will have been placed at a new