[hotfix] Update string "chk-" to constant

[spoon-lz]

What is the purpose of the change

Update string "chk-" to constant

Brief change log

N/A

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

N/A

Documentation

N/A

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[klion26]

@spoon-lz thanks for your contribution

but

We usually don't accept code "simplifications" unless they have practical benefits like performance improvements on critical code paths.

please ref to [1][2] for more information
[1] #10319
[2] https://issues.apache.org/jira/browse/FLINK-14945

[klion26]

and when contributing, we should avoid git-merge commit in history, will use git rebase