cluster: remove duplicated tasks when prune

[xhebox]

What problem does this PR solve?

Bug introduced by #2387.

The task builder b is reused without reseting. Thus FindTomestoneNodes will be executed twice, which is not expected.

Check List

Tests

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

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

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

Release notes:

Fix duplicated prompt at `tiup-cluster prune`

[ti-chi-bot[bot]]

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[ti-chi-bot[bot]]

[LGTM Timeline notifier]

Timeline:

• 2024-07-10 03:19:15.723983521 +0000 UTC m=+414052.959217621: :ballot_box_with_check: agreed by bb7133.