Is there any way to set user specific environment variables "userly" and Nixily?

NixOS configuration currently (release 23.05) only support specifying system-wide env-vars, while it seems that the Home Manager option home.sessionVariables can be activated only in the following way:

which looks like falling back to some unnecessary system-wide configurations, and for the former approach I don’t konw wether the duplicate shell configs would confict, while the latter is quite not functional.