RakuAST: make sure you can call HLL methods on $=finish

Instead of getting an error such as:
  No such method 'lines' for invocant of type 'BOOTStr'.
  Found 'lines' on type 'Cool'

src/Raku/ast/compunit.rakumod

@@ -127,7 +127,8 @@ class RakuAST::CompUnit
 
     # Replace the finish content (text after =finish) of the compilation unit.
     method replace-finish-content(Mu $finish-content) {
-        nqp::bindattr(self, RakuAST::CompUnit, '$!finish-content', $finish-content);
+        nqp::bindattr(self, RakuAST::CompUnit, '$!finish-content',
+          nqp::hllizefor($finish-content, 'Raku'));
         Nil
     }