GitHub actions: adding apt package: xindy

Author: gusthoff

.github/workflows/sphinx-books-tests.js.yml

@@ -45,6 +45,7 @@ jobs:
         texlive-fonts-extra \
         latexmk \
         texlive-xetex \
+        xindy \
         fonts-lmodern \
         fonts-open-sans \
         fonts-dejavu \