Tezt: enable debug logging level for baker in flaky test

The test "baker forked migration blocks from mumbai to alpha" is flaky, see #5080 (closed).

The baker output in this job suggests a serious problem in the baker: some consensus operations are counted twice!

• https://gitlab.com/tezos/tezos/-/jobs/3850456476#L568: bakers b1 to b4 endorse even if there is no preendorsement quorum; there is no quorum because b5 does not preendorse and the consensus threshold is set to 100%
• https://gitlab.com/tezos/tezos/-/jobs/3850456476#L794: an explicit error showing the problem

Because it seems this test works well locally, this MR simply turns on the debug logging level in the baker to hopefully understand the problem better when the test fails in the CI.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
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