[avalanche] Prevent regressions in the case that promotion happens twice for a block

Summary: This ensures that any future design changes in contenders that could trigger promotion multiple times on the same block do not overwrite any polling results that may have occurred since the first promotion.

Test Plan:

ninja check-avalanche-stakecontendercache_tests

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17983