CI: Add missing build dependencies

We need gettext and sphinx for translations and documentation,
respectively.

.github/workflows/build.yml

@@ -21,7 +21,10 @@ jobs:
     steps:
       - run: dnf --assumeyes install
               /usr/bin/cmake
+              /usr/bin/msgmerge
+              /usr/bin/msgfmt
               /usr/bin/python3
+              /usr/bin/sphinx-build-3
               python3-yui
               python3-manatools
               python3-setuptools