ci: set PYTHONUTF8=1 for UTF-8 file paths

Jenkinsfile

@@ -19,8 +19,10 @@ pipeline {
                 bat '''
                     chcp 65001 > nul
                     set PYTHONIOENCODING=utf-8
+                    set PYTHONUTF8=1
+                    set LC_ALL=en_US.UTF-8
                     pip3 install mkdocs-material --quiet 2>nul || echo ok
-                    mkdocs build --clean --site-dir site
+                    python -X utf8 -m mkdocs build --clean --site-dir site
                 '''
             }
         }