Partial rollback of https://github.com/grpc/grpc/pull/34473 for RBE as
RBE needs Ubuntu 20.04 to get clang artifacts which Google publishes.
There aren't artifacts for Ubuntu 22.04 yet so there is no point of
upgrading RBE image to 22.04.
- Add a new docker image "rbe_ubuntu2004" that is built in a way that's
analogous to how our other testing docker images are built (this gives
us control over what exactly is contained in the docker image and
ability to fine-tune our RBE configuration)
- Switch RBE on linux to the new image (which gives us ubuntu20.04-based
builds)
For some reason, RBE seems to have trouble pulling the docker image from
Google Artifact Registry (GAR), which is where our public testing images
normally live, so for now, I used a workaround and I upload a copy of
the rbe_ubuntu2004 docker image to GCR as well, and that makes RBE works
just fine (see comment in the `renerate_linux_rbe_configs.sh` script).
More followup items (config cleanup, getting local sanitizer builds
working etc.) are in go/grpc-rbe-tech-debt-2023
Esun Kim