Instead of getting the `koji_build_id` from the direct reply of
the compose request call, use the one returned in the compose
status.
The reason behind this is that composer was changed so that the
CGInitBuild call to koji is now being done by a worker and not
composer itself. This means that once the compose request call
returns, the build id is not yet known. In composer release 24,
the compose request call internally waits for the worker that
does the CGInitBuild API call, but that will be changed, and
the koji_build_id will then not be returned from the compose
request API call anymore. This prepares for that. The tests are
also adapted to simulate the new behavior.
NB: this makes composer 24 a dependency, since the build id is
taken from the ComposeStatus, which was only added there.
68309e4b5a