Fix misplaced [[no_unique_address]]

This ought to fix the MSVC build; maybe it'll fix the GCC ICE.

Author: ispeters

include/stdexec/__detail/__connect_awaitable.hpp

@@ -295,8 +295,9 @@ namespace STDEXEC
           std::destroy_at(std::addressof(__awaiter_));
         }
 
-        [[no_unique_address]] union
+        union
         {
+          [[no_unique_address]]
           __awaiter_t __awaiter_;
         };
       };