install open sans as fallback font

Author: marcoow

.github/workflows/ci.yml

@@ -38,7 +38,7 @@ jobs:
         run: |
           cargo install mdbook-pandoc --locked --version 0.7.1
           sudo apt-get update
-          sudo apt-get install -y fonts-noto calibre pdftk
+          sudo apt-get install -y fonts-noto fonts-open-sans calibre pdftk
           
           export PANDOC_VERSION=3.3
           curl -LsSf https://github.com/jgm/pandoc/releases/download/${PANDOC_VERSION}/pandoc-${PANDOC_VERSION}-linux-amd64.tar.gz | tar zxf -