Don't attempt to get cwd when printing backtrace under Miri
Don't attempt to get cwd when printing backtrace under Miri

This allows Miri to print backtraces in isolation mode

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