`elan toolchain list` refuses to list any toolchains

[eric-wieser]

See this shell session:

$ elan toolchain list
leanprover/lean4:v4.13.0-rc3
leanprover/lean4:v4.8.0-rc2
$ elan toolchain link oh--no--anyway ~/.elan/toolchains/leanprover--lean4---v4.13.0-rc3/
$ elan toolchain list
error: invalid toolchain name: 'oh/no/anyway'

Some motivation for this weird elan toolchain link command; I need a way to smuggle arbitrary paths into toolchain names, and was previously encoding path/to/my/toolchain as path--to--my--toolchain, since / characters are not legal.

This all works correctly when actually running lean and lake, but elan toolchain list fails.