cargo workspaces break `wiggle::from_witx` path resolution

when using `wiggle::from_witx` adding a crate to a workspace breaks inclusion of of `witx` files.
the wiggle [docs](https://docs.rs/wiggle/0.31.0/wiggle/macro.from_witx.html) suggest this is based on `CARGO_MANIFEST_DIR`, but i [believe](https://doc.rust-lang.org/cargo/reference/environment-variables.html#environment-variables-cargo-sets-for-crates) this should always be the package manifest directory.