Remove closure use from message::builder::Builder::build

[zeenix]

This is adding to the binary size (#304).

[zeenix]

with latest of everything, I'm not seeing this method appearing anywhere on the top in any binary I've looked anymore. 🤷

However now on the top is:

 0.7%   1.4% 48.2KiB           zbus zbus::connection::builder::Builder::build_::{{closure}}

That method is not even using any generics, so not sure what's going on. 🤷