bump backtrace
bump backtrace

https://github.com/rust-lang/backtrace-rs/pull/446 allows binaries built
with Nix on macOS to be symbolized.

graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust