x/build/env/linux-ppc64le/osuol: Update container base image to Ubuntu 20.04?

[pmur]

The current ppc64le containers have an old version of gcc which limits the test coverage of some cgo features, notably the sanitizers (#54645), when ASLR is enabled (as it is on these VMs).

I am not sure there is a reasonable way to install gcc 9 or newer on these images. They should be updated to debian bullseye or Ubuntu 20.04 (which more easily supports a newer gcc, and is derived from buster).

I propose switching the container base from the debian buster to Ubuntu 20.04 with gcc-9. Are there any objections or concerns from the go team or community?

@golang/ppc64