Revert "Pin nightly toolchain used for miri job"

This reverts commit c1826eb.

Author: dtolnay

.github/workflows/ci.yml

@@ -106,8 +106,6 @@ 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 --target ${{matrix.target}}
       - run: cargo miri test --target ${{matrix.target}} --features preserve_order,float_roundtrip,arbitrary_precision,raw_value