Support RUSTUP_TERM_COLOR as an override environment variable

[rami3l]

Closes#3433, getting my feet wet at the same time.

The variable name and the description are directly copied from the Cargo Book.

Concerns

• Is the naming of the variable ideal?
• Are there any test cases to be added for this change?
• Should we add a CLI flag (e.g. --color) for this as well?
  • If so, should we use Clap's env feature?

[rbtcollins]

Thank you for the patch.

Is the naming of the variable ideal?

I think its fine.

Is there any test cases to be added for this change?

yes, unit tests in terminalsource would be best I think.

Should we add a CLI flag (e.g. --color) for this as well?

We generally don't expose variables as CLI parameters.

[rami3l]

Hmmm. IMO that CI error has nothing to do with the changes I've made... (cc #3437)

[weihanglo]

(drive-by comment)

Some related issue in rust-lang/cargo, if we want to make sure the behavior is in sync.

• Cargo display color codes on terminals (TERM) without color support cargo#11211 (comment) (switched to std::io::IsTerminal though)
• Support CLICOLOR cargo#9835