vectorscan: don't build benchmarks

This is change is a preparation for the vectorscan version bump.

Benchmarks are not build for linux anyway, because linux uses `FAT_RUNTIME`. [1]
On darwin vectorscan 5.4.12 does not build, because building the benchmarks fails.
See comments on https://github.com/NixOS/nixpkgs/pull/427488.

[1]: https://github.com/VectorCamp/vectorscan/blob/vectorscan/5.4.12/benchmarks/CMakeLists.txt#L4

pkgs/by-name/ve/vectorscan/package.nix

@@ -58,6 +58,7 @@ stdenv.mkDerivation rec {
   # For generic builds (e.g. x86_64) this can mean using an implementation not optimized for the
   # potentially available more modern hardware extensions (e.g. x86_64 with AVX512).
   cmakeFlags = [
+    "-DBUILD_BENCHMARKS=OFF"
     (if enableShared then "-DBUILD_SHARED_LIBS=ON" else "BUILD_STATIC_LIBS=ON")
   ]
   ++ (