toolchain '$toolchain' may not be able to run on this system UX is not working well

Wanting to use musl instead of gnu, I run into this issue:

$ rustup toolchain add stable-x86_64-unknown-linux-musl
warning: toolchain 'stable-x86_64-unknown-linux-musl' may not be able to run on this system.
warning: If you meant to build software to target that platform, perhaps try `rustup target add x86_64-unknown-linux-musl` instead?
info: syncing channel updates for 'stable-x86_64-unknown-linux-musl'

  stable-x86_64-unknown-linux-musl unchanged - (error reading rustc version)

Notes

$ rustup --version
rustup 1.23.1 (2020-12-18)
info: This is the version for the rustup toolchain manager, not the rustc compiler.
info: The currently active `rustc` version is `(error reading rustc version)`

$ rustup show
Default host: x86_64-unknown-linux-gnu
rustup home:  /home/folaht/.rustup

installed toolchains
--------------------

stable-x86_64-unknown-linux-gnu
stable-x86_64-unknown-linux-musl (default)

active toolchain
----------------

stable-x86_64-unknown-linux-musl (default)
(error reading rustc version)

Reproduce rustup toolchain add stable-x86_64-unknown-linux-musl and inspect the command path that emits the host-compatibility warning and reports the rustc version error. The issue names no files or tests; done should be defined as clearer, accurate UX for this toolchain-installation case, with coverage for the reported output.