Tell user if override is set in the environment after rustup default

[das-sein]

Resolves: #1648

Given: an environment with stable, beta, and nightly installed, and an override for beta set on the current working directory.

Previous Behavior:

canis@latrans:~/rustup.rs$ rustup default nightly
info: using existing install for 'nightly-x86_64-unknown-linux-gnu'
info: default toolchain set to 'nightly-x86_64-unknown-linux-gnu'

  nightly-x86_64-unknown-linux-gnu unchanged - rustc 1.34.0-nightly (57d7cfc3c 2019-02-11)

New behavior:

canis@latrans:~/rustup.rs$ rustup default nightly
info: using existing install for 'nightly-x86_64-unknown-linux-gnu'
info: default toolchain set to 'nightly-x86_64-unknown-linux-gnu'

  nightly-x86_64-unknown-linux-gnu unchanged - rustc 1.34.0-nightly (57d7cfc3c 2019-02-11)

info: using override set by current environment
info: beta-x86_64-unknown-linux-gnu (directory override for '/home/canis/rustup.rs')

[kinnison]

Could you please find if there's a test this could go in, or else add a test for this so that we don't risk losing the functionality.

Otherwise this is nice.

[das-sein]

@kinnison No tests were affected, it seems, so I'll work on adding tests to cover this.

[das-sein]

@kinnison Added a test.

[das-sein]

@dwijnand Warning message simplified, commit squashed.

[dwijnand]

Thanks!