Allow specifying libcxx builder image.

[EricWF]

This change attempts to shift the libc++ builders over to new backend infrastructure that allows running an arbitrary container for the libc++ job.

This has been a long time in the making, and support from github and gke is finally at the point where it's possible (hopefully).

This change should also demonstrate another important property: No Downtime Upgrades.

If this goes well, we'll be able to test the upgrade as a part of the PR process, and then commiting it to main should (ideally) not break anything.