Jenkinsfile: automatically update flutter checkout

Jenkinsfile

@@ -5,6 +5,18 @@ pipeline {
     }
 
     stages {
+        stage('Update flutter checkout') {
+            steps {
+                script {
+                    sh '''#!/bin/sh
+                    set -xe
+                    cd /home/jenkins/development/flutter
+                    git pull
+                    '''
+                }
+            }
+        }
+
         stage('Build release') {
             steps {
                 script {