sys cannot be read when tidb_isolation_read_engines is set to tiflash

[YangKeao]

create table sys.t (id int);
insert into sys.t values (1),(2);
set tidb_isolation_read_engines='tiflash';
select * from sys.t;

It'll fail to select from table t.

It's actually not a big problem, because there isn't any table inside sys (the sys.schema_unused_indexes is a view on information_schema, so it works fine). However, it may cause some potential bugs in the future, so fix it asap.