Display the cargo and rustup home paths

[skilly-lily]

During interactive installation, the user will see the cargo home
directory as well as the rustup home directory. They will also see the
environment variables which control these paths.

Fixes #1907

[skilly-lily]

CI builds are failing due to some windows-only hard link error. Doesn't seem related to this change.

All errors look like this:

---- warn_about_and_remove_stray_hash stdout ----
thread 'warn_about_and_remove_stray_hash' panicked at 'called `Result::unwrap()`
on an `Err` value: Os { code: 1142, kind: Other, message: "An attempt was made to
create more links on a file than the file system supports." }',
src\libcore\result.rs:997:5

[rbtcollins]

Grammar error here - "will be installed the" -> "will be installed into the"

[skilly-lily]

Fixed. Also fixed misspelling of environment.

[rbtcollins]

This looks fine to me other than the minor grammar error. @kinnison is there anything we need to do copyright wise - DCO, just accept it, etc?

[kinnison]

If you're okay with the content then I'd like to see the commits squashed, there's no need for a "grammar fix" or "spelling fix" commit. Then it can merge.

[skilly-lily]

@kinnison @rbtcollins

• Spelling and grammar fixed
• Commits squashed

Let me know if there's anything else needed.