fix: doctest respects Cargo's color options

[ShashiSugrim]

What does this PR try to resolve?

Explain the motivation behind this change.

This commit fixes the issue where cargo's test command doesn't respect the color parameter when it gets passed in like this command: cargo t --color never --doc -- --color never

Fixes #14403

How should we test and review this PR?

Test on a basic rust project, with a file called lib.rs that looks like this

/// ```
/// bar
/// ```
pub fn foo() {}
#[cfg(test)]
mod tests {
    #[test]
    fn foo() {
        bar
    }
}

You can try to replicate the same commands @zacknewman used in the description of this issue.
cargo t --color never --doc -- --color never

You will see that compared to the official build of cargo, this build will respect the --color argument

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @weihanglo (or someone else) some time within the next two weeks.

Please see the contribution instructions for more information. Namely, in order to ensure the minimum review times lag, PR authors and assigned reviewers should ensure that the review label (S-waiting-on-review and S-waiting-on-author) stays updated, invoking these commands when appropriate:

• @rustbot author: the review is finished, PR author should check the comments and take action accordingly
• @rustbot review: the author is ready for a review, this PR will be queued again in the reviewer's queue

[ShashiSugrim]

Sorry for the problems. I am trying to fix my branch commits

[ShashiSugrim]

Hello, please review the latest changes, thank you

[weihanglo]

Just want to call out that this may break others' builds if they already have --color in their RUSTDOCFLAGS. rustdoc doesn't allow more then one occurrence of the --color flag. While we don't consider this a breaking change, this is worth mentioning in changelog.

[epage]

@bors r+

[bors]

📌 Commit 4657015 has been approved by epage

It is now in the queue for this repository.

[bors]

⌛ Testing commit 4657015 with merge 3ef3f61...

[bors]

☀️ Test successful - checks-actions
Approved by: epage
Pushing 3ef3f61 to master...