Use stable toolchain for clippy.

Dependent on `<https://github.com/rust-lang/rust-clippy/issues/11533>`
being fixed.

Author: azriel91

.github/workflows/ci.yml

@@ -77,7 +77,9 @@ jobs:
       - uses: actions/checkout@v3
       - uses: dtolnay/rust-toolchain@master
         with:
-          toolchain: nightly
+          # Normally use nightly for better lints, but we're blocked by
+          # <https://github.com/rust-lang/rust-clippy/issues/11533>
+          toolchain: stable
           components: clippy
 
       - name: 'Run clippy'