Cargo clippy on test code

src/utils/string_utils.rs

@@ -44,7 +44,9 @@ mod test {
     const VAR_ONE: &str = "VAR_ONE";
     #[test]
     fn test_apply_env_var() {
-        env::set_var(VAR_ONE, "good");
+        unsafe {
+            env::set_var(VAR_ONE, "good");
+        }
         let src = format!("This is ${{{}}}", VAR_ONE);
         assert_eq!("This is good", apply_env_vars(&src));
     }