fix CGRefundBuild to release build properly
CGInitBuild will fail on already existing build (even canceled/failed),
so it can't be used to reuse previously refunded nvr.
CGRefund build doesn't delete reservation. Having reservation token for
build which is no more in BUILDING doesn't make much sense and blocks
and reservation of same NVR.
Fixes: https://pagure.io/koji/issue/1852