Add ability to set rust version explicitly, rather than rustup latest

[bleggett]

The image build scripts always let the rustup binary choose the latest rust target version to install.

This can break things when rustup is updated and pulls a newer version of rust that the components in this repo haven't yet been updated to build against, see japaric/xargo#349 for example.

That can be fixed, but in general it would be good to allow a specific/fixed/pinned version to be specified, if desired, for whatever reason.

This PR adds the ability to request a specific rust target version by setting the RUST_VERSION container arg, such as

• RUST_VERSION=1.88

if unset, or RUST_VERSION=latest, fall back to current behavior of letting rustup decide what version to install (current behavior).

I updated the ARG default for RUST_VERSION to be latest in all the images, for the sake of explicitness.

[Emilgardis]

Thank you for the pr! However, I think I'd rather just remove xargo, it's not needed anymore with build-std working well. What do you think? see #692

[bleggett]

Thank you for the pr! However, I think I'd rather just remove xargo, it's not needed anymore with build-std working well. What do you think? see #692

Ah, missed that! That sounds better.