book.yml: Pin Rust version

1.82 introduced a regression in book tests:
rust-lang/rust#131893

.github/workflows/book.yml

@@ -27,8 +27,10 @@ jobs:
         options: --health-cmd "cqlsh --debug scylladb" --health-interval 5s --health-retries 10
     steps:
     - uses: actions/checkout@v3
+    # Set to 1.81 because of regression in 1.82: https://github.com/rust-lang/rust/issues/131893
+    # TODO: change back to latest stable after this bug is fixed.
     - name: Update rust toolchain
-      run: rustup update
+      run: rustup default 1.81
     - name: Install mdbook
       run: cargo install mdbook --no-default-features
     - name: Build the project