[resolver] Query the build group on secondary

[msimacek]

Primary and secondary should have the same build group. But they don't
have the same repos which we now use for getting the build group at
a particular point in time. So it needs to query the koji which owns the
repo, which is secondary. It is also more consistent - both repo and
build group are queried on secondary.

[codecov]

Codecov Report

Merging #248 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #248   +/-   ##
=======================================
  Coverage   82.83%   82.83%           
=======================================
  Files          41       41           
  Lines        3368     3368           
=======================================
  Hits         2790     2790           
  Misses        578      578

Impacted Files	Coverage Δ
koschei/backend/services/resolver.py	96.74% <ø> (ø)	⬆️

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