Fix a missing lock when accessing the block status

Summary:
The status is changed in the validation task, so we need to hold cs_main when accessing from the avalanche event loop.
Introduced in D12986.

Test Plan:

../contrib/teamcity/build-configurations.py build-tsan

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12990