set RUST_BACKTRACE=1 by default

[milahu]

backtrace should be default on if RUST_BACKTRACE is unset in env

crashes can be rare, hard to reproduce this change makes it easier to debug crashes

RUST_BACKTRACE=full may be too verbose by default

https://stackoverflow.com/questions/54055139/how-to-pass-rust-backtrace-1-when-running-a-rust-binary-installed-in-debian#71731489

use std::env;
fn main() {
  // this method needs to be inside main() method
  env::set_var("RUST_BACKTRACE", "1");
}