Add support for rustup target add all to install all available targets

[gnzlbg]

Closes #1653 .

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EDIT: for context, the godbolt machines try to provide all available targets, either by manually hardcoding the ones available for each toolchain, or by parsing the components list (which isn't super machine friendly), and installing them manually. This would simplify things.

I suspect that Rustup integration tests could also try to install all targets for all toolchains to discover issues (I've seen the installation of available components fail in the past). Sadly I cannot find where these integration tests live.

[kinnison]

I suspect that Rustup integration tests could also try to install all targets for all toolchains to discover issues (I've seen the installation of available components fail in the past). Sadly I cannot find where these integration tests live.

We don't integration test against the release channels by default -- That's an interesting idea to add though.

[kinnison]

or by parsing the components list (which isn't super machine friendly)

We eventually want nice JSON interaction for this kind of thing, but that's medium to long term

[kinnison]

One UX niggle, otherwise LGTM.

[liuchong]

A strange requirement... for me 🤣

[gnzlbg]

@liuchong you don't need to provide VMs that contain all rust targets for a particular nightly.. every night.. ? :D If you have to do this, keeping track of which targets are added and removed every night is a pain, and basically unmaintainable unless you try to parse the human readable output that rustup produces, and even then, because rustup does not tests that all targets are actually "installable", often you need to manually work situations where this fails.

[kinnison]

This looks good now, thank you very much!

[kinnison]

Assuming the CI passes, this can merge later today.

[gnzlbg]

CI is green!