RakuAST: un-break the REPL

The current REPL implementation loads the REPL class by running a comp
unit that's just "REPL", i.e. evaluates the class name. It does so by
stariting the compiler with :outer_ctx(nqp::null()). The old frontend
took this as a signal, that the comp unit's result is wanted. The
RakuAST frontend expected a concrete outer_ctx instead.

Fix by adopting the same criteria as the old frontend.

src/Raku/Actions.nqp

@@ -300,7 +300,7 @@ class Raku::Actions is HLL::Actions does Raku::CommonActions {
 
         # Sort out sinking; the compilation unit is sunk as a whole if we are
         # not in a REPL or EVAL context.
-        $cu.mark-sunk() unless nqp::isconcrete(%*COMPILING<%?OPTIONS><outer_ctx>);
+        $cu.mark-sunk() unless nqp::existskey(%*COMPILING<%?OPTIONS>, 'outer_ctx');
         $cu.calculate-sink();
 
         # Have check time.