show active-toolchain with rustc info

[Rustin170506]

close #2685

[kinnison]

Could you please add a comment here showing what a full output of rustup show displays (before/after your PR if it has changed) and what rustup show --verbose shows?

I'm not entirely sure it's worth adding --verbose rather than just always showing the information so I'd like to see.

[Rustin170506]

Without my change:

stable-x86_64-apple-darwin (default)

with my change but without verbose:

➜  rustup git:(rustin-patch-show) ./home/bin/rustup  show active-toolchain                                     
stable-x86_64-apple-darwin (default)
➜  rustup git:(rustin-patch-show) 

with my change and verbose:

➜  rustup git:(rustin-patch-show) ./home/bin/rustup  show active-toolchain --verbose
stable-x86_64-apple-darwin (default) - rustc 1.51.0 (2fd73fabe 2021-03-23)
➜  rustup git:(rustin-patch-show) 

[Rustin170506]

@kinnison ping~

[rbtcollins]

I think showing it in the same format that rustup show does would be better: one per line. Concretely this is better for folk using sed to query etc.

[Rustin170506]

I think showing it in the same format that rustup show does would be better: one per line. Concretely this is better for folk using sed to query etc.

Fixed.