Consolidate check-seeds builds

Summary:
Having these builds be separate doesn't make sense since they do not have differing configurations.

Consolidating them reduces the number of steps to run these checks on diffs from 2 to 1 and has the side
benefit of reducing clutter on the Teamcity dashboard.

Test Plan: Run check-seeds on this diff.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5183