ci: add zh_CN.gb18030 locale for GNU tests

Author: ChrisDryden

.github/workflows/GnuTests.yml

@@ -104,6 +104,7 @@ jobs:
         sudo locale-gen --keep-existing am_ET.UTF-8 # Ethiopia
         sudo locale-gen --keep-existing th_TH.UTF-8 # Thailand
         sudo locale-gen --keep-existing ja_JP.UTF-8 # Japan
+        sudo locale-gen --keep-existing zh_CN.gb18030 # China
 
         sudo update-locale
         echo "After:"