ci: use powershell for UTF-8 encoding

Jenkinsfile

@@ -16,12 +16,11 @@ pipeline {
 
         stage('Build Docs') {
             steps {
-                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
+                powershell '''
+                    [System.Console]::OutputEncoding = [System.Text.Encoding]::UTF8
+                    $env:PYTHONIOENCODING = 'utf-8'
+                    $env:PYTHONUTF8 = '1'
+                    pip3 install mkdocs-material --quiet 2>$null || Write-Host 'ok'
                     python -X utf8 -m mkdocs build --clean --site-dir site
                 '''
             }