[avalanche][trivial] Avoid repeated getId() during proof registration

Summary:
Just get the proofid once and use that instead.
There will be more callsites as the function is extended so better make it clean now.

Ref T1854.

Test Plan:

ninja check-avalanche

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10762