GH: Offline builds need to install imagemagick

On Ubuntu 24.04 this is no longer pre-installed

Author: mhilbrunner

.github/workflows/build_offline_docs.yml

@@ -45,7 +45,7 @@ jobs:
           echo "$VIRTUAL_ENV/bin" >> $GITHUB_PATH
           echo "VIRTUAL_ENV=$VIRTUAL_ENV" >> $GITHUB_ENV
           sudo apt update
-          sudo apt install parallel libwebp7
+          sudo apt install parallel libwebp7 imagemagick
 
       - name: Save virtualenv cache
         uses: actions/cache/save@v4