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

[rami3l]

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):

rustup/src/cli/self_update/env.nu

Lines 1 to 3 in 178c502

	if ("{cargo_bin}" not-in ($env.Path | split row (char esep))) {
	$env.Path = ($env.Path | prepend "{cargo_bin}")
	}

#3639 (comment)

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.
#3639 (comment)

Notes

No response