[avalanche] Make sure to update the block template when a tx is finalized

Summary: If the template was requested after the tx entered the mempool but before it is finalized, then it's not updated with the tx once it becomes final. This diff fixes this.

Test Plan:

./test/functional/test_runner.py abc_mining_avalanche_preconsensus

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D18487