How do I set environment variables with Cargo?

I have a dependency listed in Cargo.toml that needs a specific environment variable set. I can run export FOO=bar in bash and all works well, but for the life of me I can't figure out how to export this environment variable at compile time with Cargo. I've tried setting the environment variable in build.rs via std::env, Command, and println!, all to no effect:

// build.rs
fn main() {
    Command::new("ls")
        .env("FOO", "bar")
        .spawn()
        .expect("ls command failed to start");
}

// build.rs
fn main() {
    std::env::set_var("FOO", "bar");
}

// build.rs
fn main() {
    println!("cargo:rustc-env=FOO=bar");
}

Since Cargo 1.56, you can use the configurable-env feature via the [env] section in config.toml. This is not the same file as Cargo.toml, but it can still be set per project:

[env]
FOO = "bar"
PATH_TO_SOME_TOOL = { value = "bin/tool", relative = true }
USERNAME = { value = "test_user", force = true }

Environment variables set in this section will be applied to the environment of any processes executed by Cargo.

relative means the variable represents a path relative to the location of the directory that contains the .cargo/ directory that contains the config.toml file.

force means the variable can override an existing environment variable.

For more information about the history of this feature, see the related GitHub issue.