mergify: CI only if PR is without conflicts

[0xc0170]

Summary of changes

Conflict -> needs: work not CI

PR #12152 was a test case - I checked simulator. This fixes it, no CI if there is a conflict with base branch.

Impact of changes

Migration actions required

Documentation

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Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[X] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

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[ciarmcom]

@0xc0170, thank you for your changes.
@ARMmbed/mbed-os-maintainers please review.

[0xc0170]

CI started to progress

@adbridge Please review