book: pin to mdbook v0.4.40 for now

.github/workflows/rust.yml

@@ -110,10 +110,16 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
-      - uses: taiki-e/install-action@v2
-        with:
-          tool: mdbook
-      - run: mdbook build book
+      - name: Generate "book/theme/index.hbs" as "skeleton" of the generated pages.
+        run: ./update-theme.py
+        working-directory: book
+      - name: Install mdbook
+        run: |
+          mkdir -p $HOME/bin
+          curl --location --silent --show-error --fail https://github.com/cargo-bins/cargo-quickinstall/releases/download/mdbook-0.4.40/mdbook-0.4.40-x86_64-unknown-linux-gnu.tar.gz | tar -xzvvf - -C $HOME/bin
+      - name: Convert the book to HTML
+        run: $HOME/bin/mdbook build
+        working-directory: book
 
   DevSkim:
     name: DevSkim

book/update-theme.py

@@ -12,7 +12,7 @@ from pathlib import Path
 INDEX_HBS_DOMAIN = "api.github.com"
 INDEX_HBS_PORT = 443
 INDEX_HBS_PROTO = "GET"
-INDEX_HBS_PATH = "/repos/rust-lang/mdBook/contents/src/theme/index.hbs"
+INDEX_HBS_PATH = "/repos/rust-lang/mdBook/contents/src/theme/index.hbs?ref=v0.4.40"
 INDEX_HBS_DATA = None
 INDEX_HBS_HEADERS = {
     "user-agent": "Update index.hbs for +https://github.com/rinja-rs/rinja",