Don't mark $_ container with is dynamic in 6.d

We already changed the spectests to allow for this change to happen
during the 6.d release preparations.

The moving of `finishpad` is to make sure that we don't create the topic
container descriptor until after the language version is decided;
otherwise, even in 6.c it will be created as non-dynamic and then
re-used after that point in that mode.

src/Perl6/Grammar.nqp

@@ -824,9 +824,9 @@ grammar Perl6::Grammar is HLL::Grammar does STD {
             $*W.loading_and_symbol_setup($/)
         }
 
-        <.finishpad>
         <.bom>?
         <lang-version>
+        <.finishpad>
         <statementlist=.FOREIGN_LANG($*MAIN, 'statementlist', 1)>
 
         <.install_doc_phaser>

src/Perl6/World.nqp

@@ -1913,8 +1913,8 @@ class Perl6::World is HLL::World {
                 'default_value',   $WHAT,
                 'scalar_value',    $WHAT,
             );
-            my $desc :=
-              self.create_container_descriptor($Mu, $name, $WHAT, 1);
+            my $dynamic := $*W.lang-ver-before('d') || $name ne '$_';
+            my $desc := self.create_container_descriptor($Mu, $name, $WHAT, $dynamic);
 
             my $cont := self.build_container_and_add_to_sc(%info, $desc);