`on(sch,sndr)` should specify its attributes

[ericniebler]

on(sch,sndr) returns make-sender(on, sch, sndr), which means that the on sender has one child. [exec.adapt.general] says:

A parent sender with a single child sender sndr has an associated attribute object equal to FWD-ENV(get_env(sndr)).

however, on(sch,sndr) does not know where it will be started or where it will complete. it would be wrong for the on(sch,sndr) sender to have sndr's completion schedulers or domains.

on must define impls-for<on_t>::get-attrs to return FWD-ENV(get_env(sndr)) except without the following queries:

• get_completion_scheduler_t<Tag> for any completion tag Tag,
• get_domain_t, or
• get_domain_override_t.