Unpin nightly for miri (#11462)

# Objective

- Using the latest nightly for `miri` is generally desirable as we want
to catch regressions upstream or new problems quickly
- rust-lang/rust#120123 should be merged now, so
it should work.

## Solution

- Revert #11421.

---------

Co-authored-by: François <mockersf@gmail.com>
Co-authored-by: Alice Cecile <alice.i.cecil@gmail.com>

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ on:
 
 env:
   CARGO_TERM_COLOR: always
-  NIGHTLY_TOOLCHAIN: nightly-2024-01-17
+  NIGHTLY_TOOLCHAIN: nightly
 
 jobs:
   build: