maturin looks in the wrong path if the crate is part of a workspace

[stusmall]

Bug Description

Maturin will look in the target directory of the crate for the header rather than the target directory of the workspace.

Your Python version (python -V)

3.8.12

Your pip version (pip -V)

22.0.3

What bindings you're using

cffi

Does cargo build work?

• Yes, it works

If on windows, have you checked that you aren't accidentally using unix path (those with the forward slash /)?

• Yes

Steps to Reproduce

I wrote a minimal reproduction case for this here: https://github.com/stusmall/maturin-safer-ffi

This crate is mostly to cover the much larger issue #831 but also cover this. If you look at the xtask it creates an empty target directory and puts the processed header file there so maturin can find it. Nothing else is in that target directory, all other build artifacts are in the target directory at the base of the workspace