unpin miri (#6863)

# Objective

- rust-lang/miri#2713 was merged into miri. See if this fixes miri for bevy.

Author: hymm

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ on:
 
 env:
   CARGO_TERM_COLOR: always
-  NIGHTLY_TOOLCHAIN: nightly-2022-11-28
+  NIGHTLY_TOOLCHAIN: nightly
 
 jobs:
   build: