Fix thread safety of "foo{$x}bar"

Previously, we didn't do a closure clone of the block, which meant it
could get its outer captured by a different thread (or go very wrong
if a recursive call was done inside of the closure). Doing this clone
corrects the bug, and brings it in line with other closures, though
does cause a change to `state` inside such as block (again, bringing
it in line with how things usually work, though there was a spectest
based around the previous wrong expectation).

src/Perl6/Actions.nqp

@@ -9694,10 +9694,9 @@ class Perl6::QActions is HLL::Actions does STDActions {
         $blast.annotate('past_block', WANTALL($blast.ann('past_block'),'escape{}'));
         make QAST::Op.new(
             :op('callmethod'), :name('Stringy'),
-            WANTED(QAST::Op.new(
-                :op('call'),
-                QAST::Op.new( :op('p6capturelex'), $blast ),
-                :node($/)), 'escape{}'));
+            WANTED(
+                QAST::Op.new( :op('call'), block_closure($blast), :node($/) ),
+                'escape{}'));
     }
 
     # The next three are currently only used for tr///.