Fix CentOS 8 builds. (#377)
Fix CentOS 8 builds. (#377)

Rename build stages from rockylinux8 back to centos8. A lot of our test and release infrastructure assumes that the CentOS 8 build stages are named centos8-build and centos8. We can rename them later, with careful coordination.
GoogleCloudPlatform/ops-agent,GoogleCloudPlatform/ops-agent