Set CARGO_MANIFEST_DIR env var to manifest_dir path

[Trouv]

I've been trying this project out on one of my projects and I ran into this issue. Some of the code snippets in my book use derive macros from bevy like.

use bevy::prelude::*;
#[derive(Default, Component)]
struct ComponentA;

Apparently these macros are written to look for CARGO_MANIFEST_DIR in the env, which wasn't found when compiling with $ mdbook build book, resulting in errors like the following:

- Test: getting-started.md_line_1 (Failed to compile)
--------------- Start of Test Log:  getting-started.md_line_1 ---------------
No stdout was captured.
----- Stderr -----

error: proc-macro derive panicked
  --> /home/trouv/projects/bevy_ecs_ldtk/book/doctest_cache/keeper_u2A0TkIqkB_MFOpJSd2QQMhrWmM5g-Zm2WlddxCS79Q.rs:25:19
   |
25 | #[derive(Default, Component)]
   |                   ^^^^^^^^^
   |
   = help: message: called `Option::unwrap()` on a `None` value

Setting CARGO_MANIFEST_DIR manually seems to fix the issue for me:

$ CARGO_MANIFEST_DIR=. mdbook build book

Curious if setting this could just be a part of mdbook-keeper since I'm already providing a manifest_dir in the book.toml?