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.
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