Environment variables still accessible through /proc

Summary

### Impact Environment variables can be read from procfs unless a new process is started. ### PoC ``` use birdcage::{Birdcage, Sandbox}; use std::{env, fs}; fn main() { Birdcage::new().lock().unwrap(); assert_eq!(env::var_os("SECRET"), None); let environ = fs::read_to_string("/proc/self/environ").unwrap(); assert!(!environ.contains("SECRET"), "ENVIRON CONTAINS SECRET:\n{environ}"); } ``` ``` $ SECRET=test cargo run thread 'main' panicked at src/main.rs:10:5: ENVIRON CONTAINS SECRET: [truncated] ``` ### Possible Solutions The simplest solution would be relying on the ptrace isolation and **always** spawning a new process by changing birdcage's API to create a new command. With an additional PID namespace the guarantees could be even further reinforced.