Force the cpuid option for libutil rather than relying on detection

This is more robust, and match's Nixpkgs policy to force enable flags
statically by default (a common distro thing).

src/libutil/package.nix

@@ -102,6 +102,10 @@ mkDerivation (finalAttrs: {
     ''
   );
 
+  mesonFlags = [
+    (lib.mesonEnable "cpuid" stdenv.hostPlatform.isx86_64)
+  ];
+
   env = {
     # Needed for Meson to find Boost.
     # https://github.com/NixOS/nixpkgs/issues/86131.