[Pre-RFC] Proposal: bound Cargo's implicit upward discovery for config.toml files

Partial workaround: If you can have a directory path that is clean from unwanted configs, you can cd to it and use --manifest-path=/elsewhere/Cargo.toml to separate Cargo's config search (relative to cwd) from the location of the crate being built.

Crates invoking cargo from build.rs will unfortunately revert to their own crate's cwd.