Data races when using crate's `temp_env::with_vars_unset`

[skatromb]

Spent some time debugging problem in tests using temp_env::with_vars_unset(). Even though it's used only in one test, it could interfere with the rest tests, because uses unsafe fn std::env::set_var.

I think it's worth noting that potential problem at README.md. I may do a PR if you're okay with that.

Here is zipped example of code setup that shows the problem.

code_example.zip

Same issue happens when I use temp_env::with_var with env variable export X=value

#[cfg(test)]
mod tests {
    use std::{thread::sleep, time::Duration};

    #[test]
    fn test_unset() {
        temp_env::with_var("X", Some("blah"), || {
            sleep(Duration::from_millis(10));
            assert_eq!(std::env::var("X").unwrap(), "blah");
        });
    }

    #[test] 
    fn test_read() {
        sleep(Duration::from_millis(1));
        assert_eq!(std::env::var("X").unwrap(), "value");
    }
}

Second test test_read fails, because gets "blah"