Consistent PCP versions for container images

[portante]

As of PR #2607, the version of PCP used in container builds varies. For CentOS 7, 8, and the Fedoras we pull the latest version of PCP from JFrog artifactory. For CentOS 9, we used whatever is available.

We should consider settling on the same version across all the platforms instead of ending up with different versions.

[webbnh]

@portante, that's the wrong PR: it should be #2858. (The changes in #2607 didn't actually modify the location which the PCP RPMs were pulled from.)

At the time of #2858, PCP was available via the CRB repo for CentOS 9 Stream (and not from JFrog, IIRC), so the special handling was no longer required for that distro.

This issue begs the question of whether we would prefer Pbench to have a common version of PCP across all distributions, or whether we would rather have the version of PCP which is packaged for each distribution. I don't know how one answers that question, but, when we can, I think we prefer to pull packages for a given distro from that distro's repos....