feat(debug): log anyhow backtrace on error with RUST_[LIB_]BACKTRACE=1

With `RUST_[LIB_]BACKTRACE=1` anyhow captures backtraces
to the origin of an error chain.
The backtrace can then be accessed via `anyhow::Error::backtrace`,
which returns a `Display`able type (`std::backtrace::Backtrace`
at our version of rust).

To avoid being too noisy and allow targeted enabling of backtraces,
the backtrace is logged with `debug!` under the `flox::backtrace` target.

Unfortunately, `thiserror` supports backtraces only
when compiled with rust nightly [1][2], on account of being blocked by
the stabilization of `error_generic_member_access` [3].

[1]: <dtolnay/thiserror#390>
[2]: <dtolnay/thiserror#387>
[3]: <rust-lang/rust#99301>

Author: ysndr

cli/flox/src/main.rs

@@ -157,6 +157,8 @@ fn main() -> ExitCode {
 
             if let Some(message) = message {
                 message::error(message);
+                debug!(target: "flox::backtrace", "{}", e.backtrace());
+
                 return ExitCode::from(1);
             }
 
@@ -168,6 +170,8 @@ fn main() -> ExitCode {
 
             message::error(err_str);
 
+            debug!(target: "flox::backtrace", "{}", e.backtrace());
+
             ExitCode::from(1)
         },
     };