[seeder] Use netmagic from chainparams instead of a cached global

Summary:
See title. The global cache can cause weird behavior in unit tests
and I don't see any benefit to keeping it around.

Test Plan:

ninja bitcoin-seeder check-seeder

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8623