disttask: fix potential block on DXF scheduler (#59945)

[ti-chi-bot]

This is an automated cherry-pick of #59945

What problem does this PR solve?

Issue Number: close #59944

Problem Summary:
The channel receiver may not receive, then send a signal to the channel, which may get blocked.

What changed and how does it work?

Use select-default to discard the signal.

Check List

Tests

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

  • I checked and no code files have been changed.

Side effects

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

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

fix potential block on DXF scheduler

[ti-chi-bot]

This cherry pick PR is for a release branch and has not yet been approved by triage owners.
Adding the do-not-merge/cherry-pick-not-approved label.

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[ti-chi-bot]

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[ti-chi-bot]

@ti-chi-bot: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/build	eddb767	link	true	/test build
idc-jenkins-ci-tidb/check_dev_2	eddb767	link	true	/test check-dev2

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Please upload report for BASE (release-8.5@6f8a5f8). Learn more about missing BASE report.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-8.5     #62139   +/-   ##
================================================
  Coverage               ?   74.9536%           
================================================
  Files                  ?       1643           
  Lines                  ?     450258           
  Branches               ?          0           
================================================
  Hits                   ?     337485           
  Misses                 ?      92317           
  Partials               ?      20456           

Flag	Coverage Δ
integration	43.2261% <ø> (?)
unit	72.6804% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	52.9278% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	45.8915% <0.0000%> (?)

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