br/operator: fix adapt env for snapshot backup stuck when encountered error (#52607)

[ti-chi-bot]

This is an automated cherry-pick of #52607

What problem does this PR solve?

Issue Number: close #52846

Problem Summary:
When we failed to prepare the connections for wait apply, the channel may never be closed. Hence the pause PD operation (which relies on the finishing of the prepare connections stage) get stuck.

What changed and how does it work?

Selected over the context so it can be canceled.

Check List

Tests

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

  The change is almost trivial.

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.

None

[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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1. It must be approved by the approvers firstly.
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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
Once this PR has been reviewed and has the lgtm label, please assign yujuncen for approval. For more information see the Code Review Process.

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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/mysql-test	1cfc58e	link	true	/test mysql-test
idc-jenkins-ci-tidb/check_dev_2	1cfc58e	link	true	/test check-dev2
idc-jenkins-ci-tidb/build	1cfc58e	link	true	/test build
idc-jenkins-ci-tidb/unit-test	1cfc58e	link	true	/test unit-test
idc-jenkins-ci-tidb/check_dev	1cfc58e	link	true	/test check-dev

Full PR test history. Your PR dashboard.

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[YuJuncen]

should be picked in #53027