importctl: fix -N to actually clear keep-download flag

-N was clearing and re-setting the same bit in arg_import_flags_mask,
which is a no-op. It should clear the bit in arg_import_flags instead,
matching what --keep-download=no does via SET_FLAG().

src/import/importctl.c

@@ -1273,7 +1273,7 @@ static int parse_argv(int argc, char *argv[]) {
                         break;
 
                 case 'N':
-                        arg_import_flags_mask &= ~IMPORT_PULL_KEEP_DOWNLOAD;
+                        arg_import_flags &= ~IMPORT_PULL_KEEP_DOWNLOAD;
                         arg_import_flags_mask |= IMPORT_PULL_KEEP_DOWNLOAD;
                         break;