Produce RHEL8 tarballs in CI

[crummel]

We don't currently produce RHEL8 tarballs - there are some differences here like the lack of system libunwind. We should use a supported RHEL8 Docker image to do these builds in CI - not like the snapshot RHEL7 image we use.

[omajid]

Using CentOS 8 or CentOS Stream - a rolling preview of what goes in the next release of RHEL 8 - might also be a good idea.

[dagood]

Triage notes:
The cross-distro tarball work #1412 would fix the direct issue here, but testing on CentOS 8 or Stream could be valuable to catch issues with dependencies.

[crummel]

[Scrub] We now have CentOS Stream 8 CI.