Patch `centpkg-sig` config

[mfocko]

Currently the centpkg-sig config assumes the git forge to be git.centos.org, this is however in contradiction to the additional check of existence here. Since Packit plans on supporting CBS Koji builds for the SIGs on GitLab, and the check is already present for the centpkg (official CentOS repositories), it makes sense to force this requirement.

TODO:

• [ ] Take config from https://git.centos.org/centos/centpkg/pull-request/99
• [ ] Make sure it replaces the official centpkg-sig config in the service and workers, maybe sandcastle?

[mfocko]

Fixed in 698aad5db4cb4f10cff7a95e44a770b223de4edc lol