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

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes are missing coverage. Please review.

❗ No coverage uploaded for pull request base (release-6.5@727a2c7). Click here to learn what that means.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-6.5     #52969   +/-   ##
================================================
  Coverage               ?   73.6439%           
================================================
  Files                  ?       1096           
  Lines                  ?     351581           
  Branches               ?          0           
================================================
  Hits                   ?     258918           
  Misses                 ?      76014           
  Partials               ?      16649           

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: BornChanger, YuJuncen

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [YuJuncen]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-05-06 06:51:59.240346472 +0000 UTC m=+858472.997482046: ☑️ agreed by BornChanger.
• 2024-05-06 06:52:57.441486579 +0000 UTC m=+858531.198622137: ☑️ agreed by YuJuncen.