book.yml: Set default toolchain globally

setup-rust-toolchain sets installed toolchain as default only for
current directory. `mdbook test` creates a dir in /tmp and executes
`rustdoc` there, leading to issues due to different toolchains being
used. Until it is fixed in the action or in mdbook, we need to manually
set stable toolchain as global default.

Author: Lorak-mmk

.github/workflows/book.yml

@@ -31,6 +31,11 @@ jobs:
       with:
         # We can't use cache here, or mdbook will complain about multiple candidates for dependency.
         cache: false
+    # This is a workaround for https://github.com/actions-rust-lang/setup-rust-toolchain/issues/79
+    # and https://github.com/rust-lang/mdBook/issues/2501
+    # When either mdbook or setup-rust-toolchain fix the issue, we can remove this.
+    - name: Set default toolchain globally
+      run: rustup default stable
     - name: Install mdbook
       uses: taiki-e/install-action@v2
       with: