Reopened backport PR failed task will block auto merging

[Mugurell]

Expected Behavior

• @Mergifyio backport creates a new PR.
• mergify doesn't automatically deletes it's backport branches

Actual Behavior

• @Mergifyio backport seems set on reusing the already open PR and branch.
• mergify automatically deletes it's backport branches (not sure this was caused our issue)

Steps to Reproduce the Problem

1. @Mergifyio backport
2. Close the Mergifyio created backport PR to restart our CI tasks
   -> mergify bot deleted the mergify/bp/... branch
3. The rule doesn't match anymore . This action has been cancelled.

Specifications

• Pull Request URL: For #6528 - Evaluate Nimbus experiments in place (backport #6575) mozilla-mobile/focus-android#6637
• Mergify Config URL: https://github.com/mozilla-mobile/focus-android/blob/main/.mergify.yml