Set mdbook version as env var

.github/workflows/main.yml

@@ -15,9 +15,11 @@ jobs:
         rustup toolchain install nightly -c rust-docs
         rustup default nightly
     - name: Install mdbook
+      env:
+        MDBOOK_VER: v0.4.3
       run: |
         mkdir bin
-        curl -sSL https://github.com/rust-lang/mdBook/releases/download/v0.3.5/mdbook-v0.3.5-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=bin
+        curl -sSL https://github.com/rust-lang/mdBook/releases/download/${{ env.MDBOOK_VER }}/mdbook-${{ env.MDBOOK_VER }}-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=bin
         echo "##[add-path]$(pwd)/bin"
     - name: Report versions
       run: |