Bump backtrace-rs

This pulls in https://github.com/rust-lang/backtrace-rs/pull/376, which
fixes Miri support for `std::backtrace::Backtrace`.

diff --git a/library/backtrace b/library/backtrace
index 893fbb23688e98376e54c26b59432a2966a8cc96..a6dd47bd588c882e735675a1379d2b61719fa380 160000 (submodule)
--- a/library/backtrace
+++ b/library/backtrace
@@ -1 +1 @@
-Subproject commit 893fbb23688e98376e54c26b59432a2966a8cc96
+Subproject commit a6dd47bd588c882e735675a1379d2b61719fa380