2023-10-15

prelude/rust: Process CARGO_MANIFEST_DIR during rustdoc test

Summary:
Rustdoc tests were not getting the `CARGO_MANIFEST_DIR` processing that
we do for all other compilations.

Reviewed By: dtolnay

Differential Revision: D50303301

fbshipit-source-id: 1c3171e7eccc2d1a0461aa93b65ff82ac880c8c0