ci: temp. pin nightly to avoid ICE

There's an unfixed issue with nightly rust that produces an ICE building
the project w/ nightly. Pin a working version until its fixed upstream.

Author: cpu

.github/workflows/ci.yml

@@ -155,7 +155,8 @@ jobs:
     - name: Install Rust
       uses: dtolnay/rust-toolchain@master
       with:
-        toolchain: ${{ matrix.toolchain }}
+        # TODO(XXX): Revert to "matrix.toolchain" after rust-lang/rust#125474 is fixed.
+        toolchain: ${{ matrix.toolchain == 'nightly' && 'nightly-2024-05-22' || matrix.toolchain }}
     - name: Run cargo check
       run: cargo check --all-targets
     - name: Run the tests