Fix ps_engine.raftlog unset under default raft-engine dir

[CalvinNeo]

What is changed and how it works?

Issue Number: Close #xxx

See tiflash#9727

The previous PR #367 does not totally fixed the issue.

Infer the dir of raft-engine, if it is not provided.

The final files under flash/raft-engine should be:

>  ll ${TIFLASH_DATA_DIR}/flash/raft-engine
total 0
-rw-r--r--. 1 tidb tidb 0 Dec 16 17:02 ps_engine
-rw-r--r--. 1 tidb tidb 0 Dec 16 17:12 ps_engine.raftlog

What's Changed:

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Release note

[JaySon-Huang]

LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JaySon-Huang

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-12-17 02:48:18.118924146 +0000 UTC m=+925088.207726688: ☑️ agreed by JaySon-Huang.