Use a variable for the mdbook version.

Author: ehuss

Diff for: .github/workflows/docs.yml

@@ -12,14 +12,16 @@ jobs:
     permissions:
       contents: write  # for Git to git push
     runs-on: ubuntu-latest
+    env:
+      MDBOOK_VERSION: 0.4.28
     steps:
     - uses: actions/checkout@v3
       with:
         fetch-depth: 0
     - name: Install mdbook
       run: |
         mkdir mdbook
-        curl -Lf https://github.com/rust-lang/mdBook/releases/download/v0.4.28/mdbook-v0.4.28-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=./mdbook
+        curl -Lf https://github.com/rust-lang/mdBook/releases/download/v${MDBOOK_VERSION}/mdbook-v${MDBOOK_VERSION}-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=./mdbook
         echo `pwd`/mdbook >> $GITHUB_PATH
     - name: Deploy docs
       run: |