[refactor] access mapBlockIndex via function in Avalanche code

Summary:
incoming PR16194 is encapsulating that global within the BlockManager
object, this part of the code is still accessing it directly, so I
changed it to use the function here, as not to make the backport
needlessly more complicated to review

Test Plan:

ninja check-avalanche

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6965