Run signing tasks on a single distro

We've had errors with the Notary client when running on newer (Ubuntu 22.04) and older (RHEL 7) distros for different reasons related to dependencies. We can both fix this and make it easier to reason about what the Notary client is doing by running the push and crypt_push tasks on a single, known-good distro (probably rhel80-small?) regardless of what distro the buildvariant is using by default.