lapack: force a rebuild on x86_64-darwin

I have no idea why, but Hydra cached a corrupted binary
/nix/store/h0412qmi29ivgrc42lfhi1x290d6l429-lapack-3/lib/liblapack.dylib
so rebuilding it fixes issues in various dependants, e.g.
https://hydra.nixos.org/log/nqfq62pqbbmsq08kb2pljdkz9ldyr57b-python3.10-numpy-1.24.2.drv

pkgs/build-support/alternatives/lapack/default.nix

···
     implementation = lapackImplementation;
   };
 
+  # TODO: drop this forced rebuild, as it was needed just once.
+  rebuild_salt = if stdenv.isDarwin && stdenv.isx86_64 then "J4AQ" else null;
+
   dontBuild = true;
   dontConfigure = true;
   unpackPhase = "src=$PWD";