node: Do not set `RUST_BACKTRACE` — heartwood

Even though it helps diagnosing panics, it should be up to the user to set RUST_BACKTRACE.

Frankly, I think it was a mistake to have radicle-node modify its own environment. We should have told the users that were seeing panics to please set RUST_BACKTRACE=1.

 }
 fn main() {
     // If `RUST_BACKTRACE` does not have a value, then we set it to capture
     // backtraces for better debugging, otherwise we keep the environments
     // value.
     const RUST_BACKTRACE: &str = "RUST_BACKTRACE";
     if std::env::var_os(RUST_BACKTRACE).is_none() {
         std::env::set_var(RUST_BACKTRACE, "1");
     }
     let options = parse_options().unwrap_or_else(|err| {
         // The lexopt errors read nicely with a comma.
         eprintln!("Failed to parse options, {err:#}");