Revert "Pin nightly toolchain used for miri job"

This reverts commit 1091cc2.

Author: dtolnay

.github/workflows/ci.yml

@@ -210,8 +210,6 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@miri
-        with:
-          toolchain: nightly-2025-05-16  # https://github.com/rust-lang/miri/issues/4323
       - run: cargo miri setup
       - run: cargo miri test
         env: