Using include-first in workspace members

When building a workspace, the working directory during build will be root directory. When building member crates `src/{}` will point to `root/src/{}` instead of `root/member/src/{}` [here](https://github.com/google/include-first/blob/main/src/lib.rs#L50).

I suggest changing this line to something like this:
```rust
let filename = Path::new(&std::env::var("CARGO_MANIFEST_DIR").unwrap()).join("src").join(filename.trim_matches('"'));
```

`CARGO_MANIFEST_DIR` always points to the directory of the package currently being built.