Regression in CWD for doctests

[jan-auer]

Problem
cargo 1.50.0-nightly changes the current working directory for doctests when executed from a workspace. Instead of the crate's root, the workspace's root is assumed. The regression originates in #8954. I think, the cwd should still point to the crate root in all cases.

Steps

1. Crate a workspace with a member crate in a subdirectory.
2. Write a doctest that reads a file relative to the crate root, e.g. src/lib.rs.
3. Test with cargo +stable test --workspace and observe the test pass.
4. Test with cargo +nightly test --workspace and observe the test fail.

Possible Solution(s)

• Revert Run rustdoc doctests relative to the workspace #8954. I have not checked if there are any follow-ups.
• Explicitly set the CWD while also somehow preserving workspace-relative file path information. It seems like this may be required to solve Rustdoc doctests should have their file/lines remapped when using -Zinstrument-coverage rust#79417.

Notes

Output of cargo version: cargo 1.50.0-nightly (d274fcf86 2020-12-07)

Example src/lib.rs for a workspace member that will pass on stable but fail on nightly:

//! ```
//! assert_eq!(std::fs::read_to_string("src/lib.rs").unwrap(), include_str!("lib.rs"));
//! ```