Proposal: `rustup env <shell>` to source the `env.*` file automatically for a certain shell

Problem you are trying to solve

... say, I suddenly decided to start using nu but that happens after my last rustup self update. In this case I'll definitely end up without any env.nu and have to manually populate that file from the repo (we shouldn't assume that I can find that file that easily in the first place):

https://github.com/rust-lang/rustup/blob/178c502bfab89b68d7e2f2fa681e07ccef295656/src/cli/self_update/env.nu#L1-L3
https://github.com/rust-lang/rustup/issues/3639#issuecomment-2542661654

Solution you'd like

... if the user wants to add another shell to the mix, manually sourcing the corresponding env.* file will do, and it's usually just a single line in their *shrc file.
OTOH, if they want to source the env.* file automatically, rustup env <shell> would be perfect in terms of adding these sourcing scripts on demand.
https://github.com/rust-lang/rustup/issues/3639#issuecomment-2545551436

Notes

No response

Start by reviewing src/cli/self_update/env.nu and the linked discussion about generating shell-specific environment files. Define the behavior and supported-shell scope for rustup env , including what automatic sourcing should mean; done requires an agreed design and corresponding command behavior.