ebs br: always save checkpoint to the warmup folder (#5507)

[ti-chi-bot]

This is an automated cherry-pick of #5507

What problem does this PR solve?

Fixed #5506.

What is changed and how does it work?

The checkpoint file is stored at the work directory where the warmup binary runs. And in most cases, it is / and won't be persisted to a PV.
This PR changes the warmup_step script, make it always set checkpoint to the directory to be warmed up(Which is probably a real EBS disk).
(Also notice that, it is impossible to pass a glob pattern to warmup_step anymore.)

Code changes

• Has Go code change
• Has CI related scripts change

Tests

• Unit test
• E2E test
• Manual test

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release Notes

Please refer to Release Notes Language Style Guide before writing the release note.

Fixed a bug that may cause the warmup checkpoint is unavailable.

[ti-chi-bot]

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[BornChanger]

/retest

[codecov-commenter]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (d67387a) 61.56% compared to head (ab67e17) 61.06%.

Additional details and impacted files

@@               Coverage Diff               @@
##           release-1.5    #5522      +/-   ##
===============================================
- Coverage        61.56%   61.06%   -0.51%     
===============================================
  Files              228      241      +13     
  Lines            29174    33127    +3953     
===============================================
+ Hits             17962    20230    +2268     
- Misses            9443    11035    +1592     
- Partials          1769     1862      +93     

Flag	Coverage Δ
e2e	23.22% <ø> (?)
unittest	61.57% <80.00%> (+<0.01%)	⬆️