File based warmup checkpoint is unavailable if the root directory isn't a PV

[YuJuncen]

Bug Report

What version of Kubernetes are you using?
N/A

What version of TiDB Operator are you using?
N/A

What storage classes exist in the Kubernetes cluster and what are used for PD/TiKV pods?
N/A

What's the status of the TiDB cluster pods?
N/A

What did you do?
Run EBS Restore, with hybrid warmup method, delete the warmup pod.

What did you expect to see?
The new warmup pod should be able to continue from the last checkpoint.

What did you see instead?
It runs from the time.Now(). (i.e. will warmup all files.)