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

@@ -46,7 +46,8 @@ jobs:
       - name: Install rust toolchain
         uses: dtolnay/rust-toolchain@master
         with:
-          toolchain: ${{ matrix.toolchain }}
+          # TODO(XXX): Revert to "matrix.rust" after rust-lang/rust#125474 is fixed.
+          toolchain: ${{ matrix.rust == 'nightly' && 'nightly-2024-05-22' || matrix.rust }}
       - name: cargo doc (all features)
         run: cargo doc --all-features --document-private-items
         env: