Pinning codecov to fix zizmor warnings

[amoghrajesh]

After fixing the upgrade checks on main, it will fail some zizmor checks, example: https://github.com/apache/airflow/actions/runs/15095677291/job/42430124052

 INFO audit: zizmor: 🌈 completed .github/workflows/stale.yml
 INFO audit: zizmor: 🌈 completed .github/workflows/test-providers.yml
error[: unpinned action reference
  --> .github/actions/post_tests_success/action.yml:47:7
   |
47 |       uses: codecov/codecov-action@v4
   |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ action is not pinned to a hash (required by blanket policy)
   |
   = note: audit confidence → High

10 findings (9 suppressed): 0 unknown, 0 informational, 0 low, 0 medium, 1 high

Earlier we used codecov/codecov-action@v4, replacing it now with pertaining hash: codecov/codecov-action@b9fd7d1

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