fix(exec): any_sender completion signatures for sequence with 3+ senders

When exec::sequence has 3+ senders, it recursively computes completion
signatures by peeling off the first sender and querying the tail as a
sub-sequence (__seq::__sndr). This tail holds child senders by const&.
The get_completion_signatures discovery machinery passes the sender type
un-decayed (as const any_sender&), but any_sender's constraint
std::derived_from<Self, _interface_> fails because reference types are
never derived_from anything.

Fix by decaying Self before the derived_from check in
_isender::_interface_::get_completion_signatures().

Fixes #2101

Author: alwaysprince05

include/exec/any_sender_of.hpp

@@ -514,7 +514,8 @@ namespace experimental::execution
         using sender_concept = STDEXEC::sender_tag;
         using _base_t::_base_t;
 
-        template <__std::derived_from<_interface_> Self, class... Env>
+        template <class Self, class... Env>
+          requires std::derived_from<std::remove_cvref_t<Self>, _interface_>
         static consteval auto get_completion_signatures()
         {
           // throw if Env does not contain the queries needed to type-erase the receiver: