usage of document-features crate breaks sccache-dist

[lissyx]

cf https://crates.io/crates/document-features

wgpu-core crate recently landed on mozilla-central making use of that, and this depends on Cargo.toml being available.

It blows up with:

 0:04.92    Compiling wgpu-core v0.19.0 (https://github.com/gfx-rs/wgpu?rev=a5c0181c3a6b4b197dcae34591dfe78bf45338b9#a5c0181c)
 0:24.78 error: Can't open Cargo.toml: Os { code: 2, kind: NotFound, message: "No such file or directory" }
 0:24.78  --> /home/alex/codaz/Mozilla/gecko-cinnabar/third_party/rust/wgpu-core/src/lib.rs:6:10
 0:24.78   |
 0:24.78 6 | #![doc = document_features::document_features!()]
 0:24.78   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 0:24.78   |
 0:24.78   = note: this error originates in the macro `document_features::document_features` (in Nightly builds, run with -Z macro-backtrace for more info)

[lissyx]

#2124

[sylvestre]

Pr welcome!