[avalanche] Use an ordered container for stakes in the proof builder

Summary:
Follow-up of D10076, this ensures the proof builder stakes are always consistent and makes the code more robust.

Ref T1676.
Depends on D10076.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1676

Differential Revision: https://reviews.bitcoinabc.org/D10077