Allow specifying a channel for the system rustc

[jflatow]

This is useful if you want to use e.g. {:system, "+nightly"}

[evnu]

The format "+nightly" might come as a surprise to users: If they use something like {:system, "nightly"}, cargo will think that the channel is actually an unknown command. We should either document the expected format with + or use atoms and translate ({:system, :nightly}). What do you think? Documenting the required + is probably the simplest thing to do here.

[jflatow]

Added some clarification, let me know what you think.

[evnu]

I like it, thanks!

[evnu]

@jflatow Can you fix the format error?

[jflatow]

Should be passing CI now, though I don't see the checks running at all again

[evnu]

Should be passing CI now, though I don't see the checks running at all again

Seems that I need to run that explicitly. CI went through, thank you!