`let_value` is not applying `FWD-ENV` as it should

[ericniebler]

The child senders should only see the parent's queries if they are forwarding queries.

Proposed resolution

Change [exec.let] para 6 as follows:

6. Let receiver2 denote the following exposition-only class template:

   namespace std::execution {
     ... as before ...
   }

   Invocation of the function receiver2::get_env returns an object e such that

   1. decltype(e) models queryable and

   2. given a query object q, the expression e.query(q) is expression-equivalent
      to env.query(q) if that expression is valid,; otherwise,
      if the type of q satisfies forwarding-query, e.query(q)
      is expression-equivalent to get_env(rcvr).query(q); otherwise, e.query(q)
      is ill-formed.

Change para 8 as follows:

8. Let Sigs be a pack of the arguments to the completion_signatures
   specialization named by completion_signatures_of_t<child-type<Sndr>,
FWD-ENV-T(env_of_t<Rcvr>)>. Let LetSigs be a pack of those types in Sigs with a return
   type of decayed-typeof<set-cpo>. Let as-tuple be an alias template such that
   as-tuple<Tag(Args...)> denotes the type decayed-tuple<Args...>. Then
   args_variant_t denotes the type variant<monostate, as-tuple<LetSigs>...>
   except with duplicate types removed.