Make rustup works also on read-only filesystem (at least for storage)

Describe the problem you are trying to solve

It would be interesting to have a way to rustup works also with read-only directory.
In particular it would be interesting to have rustup use the big files (compiler/toolchain) from a read-only directory while the configuration file would need to stay in a different (writable) directory.

Describe the solution you'd like

If we could add a env variable to point rustup to either: the configuration file or the "storage directory".
Both solution will work, even if the most interesting would be to have two variables, one for the configuration file and the other for the storage directory.

Notes

I briefly discussed this request on reddit with @danielsilverstone-ct but I didn't have time to actually put it as github request, anyhow, references here: https://www.reddit.com/r/rust/comments/d1qxj4/rustup_1190_released/ezq6fvj/

Why it is usefull
Complex distribution/installation models rely on provide a shared read-only directory where common software/files are installed.
This is very common in HEP (High Energy Physic) research, but also in HPC in general.

In am not aware of any open-internet server that allow this approach to anybody, but I am in the (slow) process of setting up one myself and I could provide an example of how this kind of systems work. Indeed are not very widespread in the industry.

Start by tracing how rustup locates its configuration file and toolchain or compiler storage directory. Evaluate separate environment-variable locations for writable configuration and read-only storage, then verify that common toolchain use works with storage on a read-only filesystem while configuration remains writable.