Robustifies jenkins env vars

Jenkinsfile

@@ -16,6 +16,9 @@ pipeline {
     PROJECT_DIR = "qt6-after/qt6-migrated-benchmark"
     BUILD_DIR = "build"
     CHANGE_TARGET = "${env.CHANGE_TARGET ?: 'origin/main'}"
+    QT_QPA_PLATFORM = "offscreen"
+    LANG = "C.UTF-8"
+    LC_ALL = "C.UTF-8"
   }
 
   stages {
@@ -41,6 +44,9 @@ pipeline {
     stage('Qt Diagnostics') {
       steps {
         sh '''
+          echo "QT_QPA_PLATFORM=$QT_QPA_PLATFORM"
+          echo "LANG=$LANG"
+          echo "LC_ALL=$LC_ALL"
           dpkg -l | grep -E "qt6-base-dev|qtbase5-dev|qt6-tools" || true
           pkg-config --list-all | grep -i qt || true
         '''
@@ -67,7 +73,7 @@ pipeline {
 
     stage('Test') {
       steps {
-        sh 'ctest --test-dir ${BUILD_DIR} --output-on-failure'
+        sh 'QT_QPA_PLATFORM=offscreen LANG=C.UTF-8 LC_ALL=C.UTF-8 ctest --test-dir ${BUILD_DIR} --output-on-failure'
       }
     }
   }