rocker/r-ver:4.0.0-ubuntu18.04 can't be built now

[eitsupi]

The changes discussed in #390 do not seem to work on ubuntu:bionic.

https://github.com/rocker-org/rocker-versioned2/runs/6126166221?check_suite_focus=true

#12 6.908 update-alternatives: using /usr/lib/x86_64-linux-gnu/openblas/libblas.so.3 to provide /usr/lib/x86_64-linux-gnu/libblas.so.3 (libblas.so.3-x86_64-linux-gnu) in auto mode
#12 6.911 update-alternatives: using /usr/lib/x86_64-linux-gnu/openblas/liblapack.so.3 to provide /usr/lib/x86_64-linux-gnu/liblapack.so.3 (liblapack.so.3-x86_64-linux-gnu) in auto mode
#12 6.914 Setting up libopenblas-dev:amd64 (0.2.20+ds-4) ...
#12 6.919 update-alternatives: using /usr/lib/x86_64-linux-gnu/openblas/libblas.so to provide /usr/lib/x86_64-linux-gnu/libblas.so (libblas.so-x86_64-linux-gnu) in auto mode
#12 6.921 update-alternatives: using /usr/lib/x86_64-linux-gnu/openblas/liblapack.so to provide /usr/lib/x86_64-linux-gnu/liblapack.so (liblapack.so-x86_64-linux-gnu) in auto mode
#12 6.925 Processing triggers for libc-bin (2.27-3ubuntu1.5) ...
#12 6.955 update-alternatives: error: alternative /usr/lib/x86_64-linux-gnu/openblas-pthread/libblas.so.3 for libblas.so.3-x86_64-linux-gnu not registered; not setting
#12 ERROR: process "/bin/sh -c /rocker_scripts/setup_R.sh" did not complete successfully: exit code: 2

@cboettig @noamross Do you think it is acceptable to end support for running scripts on Ubuntu 18.04 without fixing this issue?
My understanding is that only the R 4.0.0 images have the ubuntu18.04 flavor because it was just after the Ubuntu 20.04 release at the time and RSPM binaries were not provided for Ubuntu 20.04, etc., so it has outlived its role.

[cboettig]

yes, I favor discontinuing Ubuntu 18.04 support. (though I Think the above is fixable)

Correct, it was only introduced a something of a stop-gap measure for things that didn't have Ubuntu 20.04 compatibility at the time (for RSPM and a few other edge cases in geospatial iirc) and otherwise didn't really fit into the standard release model, and as you know it was never extended to the whole stack either.