[actions.pm] construct twigilled variables right

In <http://rt.perl.org/rt3/Ticket/Display.html?id=67846>, :$.bar
turned into $1bar. The <twigil> subrule was called with a
postfix:<?>, turning its submatch into an array.  The colonpair
action method now takes this into account. This is a first step
towards fixing #67846. Rakudo still doesn't conform to the semantics
explained in L<S06/Attributive parameters>, but now it at least has a
chance.

src/parser/actions.pm

@@ -2919,7 +2919,7 @@ method colonpair($/, $key) {
         if $<desigilname><longname> {
             $pair_key := PAST::Val.new( :value( ~$<desigilname> ) );
             $pair_val := PAST::Var.new(
-                :name( ~$<sigil> ~ ~$<twigil> ~ $<desigilname> )
+                :name( ~$<sigil> ~ ~$<twigil>[0] ~ $<desigilname> )
             );
         }
         else {