Jenkinsfile: Update deprecated 'stage' syntax

Jenkinsfile

@@ -1,10 +1,12 @@
+#!/usr/bin/env groovy
 properties([[$class: 'BuildDiscarderProperty', strategy: [$class: 'LogRotator', artifactDaysToKeepStr: '30', artifactNumToKeepStr: '10']]])
 
 node('master') {
-    stage 'Pull changes'
+    stage('Pull changes') {
         checkout scm
+    }
 
-    stage 'Build data'
+    stage('Build data') {
         sh 'mkdir -p build'
         dir('build') {
             sh '''
@@ -14,8 +16,10 @@ node('master') {
                 DESTDIR=. make install
             '''
         }
+    }
 
-    stage 'Archive data'
+    stage('Archive data') {
         sh 'rm -f data.zip'
         zip zipFile: 'data.zip', archive: true, dir: 'build/install'
+    }
 }