Incorrectly removed std::env

Reinstate  std::env into tpm.rs, which was mistakenly removed
in main commit for PR #82

src/tpm.rs

@@ -8,6 +8,7 @@ use flate2::Compression;
 use keylime_error;
 use openssl::sha;
 use serde_json::Value;
+use std::env;
 use std::fs::File;
 use std::io::prelude::*;
 use std::io::BufWriter;