taskflow: use C++20 standard

> Although Taskflow supports primarily C++17, you can enable C++20
> compilation through -std=c++20 to achieve better performance due to
> new C++20 features. That being said, we are actively working on
> migrating Taskflow to C++20, which is planned for the next major
> release v4.

pkgs/by-name/ta/taskflow/package.nix

@@ -36,6 +36,8 @@ stdenv.mkDerivation (finalAttrs: {
   ];
 
   cmakeFlags = [
+    # FIXME remove once Taskflow is updated to 4.0.0
+    (lib.cmakeFeature "CMAKE_CXX_STANDARD" "20")
     # building the tests implies running them in the buildPhase
     (lib.cmakeBool "TF_BUILD_TESTS" finalAttrs.finalPackage.doCheck)
   ];