Ignore fallback methods when looking for CUSTOM-DISPATCHER in dispatch

When looking for a CUSTOM-DISPATCHER method on an object that is called, we
want to ignore fallback methods (like FALLBACK or the junction autothreader),
as those may give us an answer, even if there is no suitable CUSTOM-DISPATCHER
method available, leading to very confusing errors.

src/vm/moar/dispatchers.nqp

@@ -2627,7 +2627,7 @@ nqp::dispatch('boot-syscall', 'dispatcher-register', 'raku-invoke', -> $capture
     }
 
     elsif nqp::isconcrete(
-        my $custom-dispatcher := nqp::decont(nqp::how_nd($code).find_method($code, 'CUSTOM-DISPATCHER'))
+        my $custom-dispatcher := nqp::decont(nqp::how_nd($code).find_method(nqp::decont($code), 'CUSTOM-DISPATCHER', :no_fallback))
     ) {
         nqp::dispatch('boot-syscall', 'dispatcher-delegate',
             nqp::unbox_s($custom-dispatcher($code)), $capture);