workloadrepo: when `WORKLOAD_SCHEMA` exists, BR failed to ignore it

[bb7133]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. enable 'workload repository':

tidb> set global tidb_workload_repository_dest='table';
tidb> admin create workload snapshot;
tidb> show databases;
+--------------------+
| Database           |
+--------------------+
| INFORMATION_SCHEMA |
| METRICS_SCHEMA     |
| PERFORMANCE_SCHEMA |
| WORKLOAD_SCHEMA    |
| mysql              |
| sys                |
| test               |
+--------------------+
7 rows in set (0.01 sec)

2. Make a full backup

bin/br backup full --storage "file:///tmp/br"

3. Make a full restore

bin/br restore full --storage "file:///tmp/br"

or

bin/br restore full --with-sys-table --storage "file:///tmp/br"

2. What did you expect to see? (Required)

Restore successfully.

3. What did you see instead (Required)

Restore failed:

Error: user db/tables: WORKLOAD_SCHEMA.HIST_SNAPSHOTS, WORKLOAD_SCHEMA.HIST_CLIENT_ERRORS_SUMMARY_BY_HOST, WORKLOAD_SCHEMA.HIST_CLIENT_ERRORS_SUMMARY_BY_USER, WORKLOAD_SCHEMA.HIST_CLIENT_ERRORS_SUMMARY_GLOBAL, WORKLOAD_SCHEMA.HIST_TIKV_REGION_STATUS, WORKLOAD_SCHEMA.HIST_PROCESSLIST, WORKLOAD_SCHEMA.HIST_DATA_LOCK_WAITS, WORKLOAD_SCHEMA.HIST_TIDB_TRX, WORKLOAD_SCHEMA.HIST_MEMORY_USAGE, WORKLOAD_SCHEMA.HIST_DEADLOCKS, ...: [BR:Restore:ErrRestoreNotFreshCluster]cluster is not fresh

4. What is your TiDB version? (Required)

(master only since 'workload repository' is not released yet)
c199ddf