Build mdbook in remark workflow

This is just to ensure that the book builds all time to not get in trouble when syncing with rust-lang/rust

.github/workflows/remark.yml

@@ -26,10 +26,19 @@ jobs:
     - name: Install remark
       run: npm install remark-cli remark-lint remark-lint-maximum-line-length remark-preset-lint-recommended remark-gfm
 
+    - name: Install mdbook
+      run: |
+        mkdir mdbook
+        curl -Lf https://github.com/rust-lang/mdBook/releases/download/v0.4.18/mdbook-v0.4.18-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=./mdbook
+        echo `pwd`/mdbook >> $GITHUB_PATH
+
     # Run
     - name: Check *.md files
       run: git ls-files -z '*.md' | xargs -0 -n 1 -I {} ./node_modules/.bin/remark {} -u lint -f > /dev/null
 
+    - name: Build mdbook
+      run: mdbook build book
+
   # These jobs doesn't actually test anything, but they're only used to tell
   # bors the build completed, as there is no practical way to detect when a
   # workflow is successful listening to webhooks only.