GitHub actions: Update checkout version and use safe.directory

[adbridge]

Summary of changes

GitHub actions is failing due to not finding directories. This change fixes this by upgrading the checkout to v3 and using
safe.directory.

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]

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

[0xc0170]

Let's fix python tests (same as it's on master), it should make them green again

[0xc0170]

CI started