[seeder] Use netmagic from chainparams instead of a cached global
See title. The global cache can cause weird behavior in unit tests
and I don't see any benefit to keeping it around.
ninja bitcoin-seeder check-seeder
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D8623