带有sh定义变量的Jenkins管道“when”条件

pipeline {
    agent { label 'php71' }
    stages {
        stage('Prepare CI...') {
            steps{
                sh '''
                    # Get the comment that was made on the PR
                    COMMENT=`echo $payload | jq .comment.body | tr -d '"'`
                    if [ "$COMMENT" = ":repeat: Jenkins" ]; then
                        BUILD="build"
                    fi
                '''
            }
        }
        stage('Build Pre Envrionment') {
            agent { label 'php71' }
            when {
                expression { return $BUILD == "build" }
            }
            steps('Build') {
                sh '''
                    echo $BUILD
                    echo $COMMENT
                '''
            }
        }
    }
}

这给了我一个错误: groovy.lang.MissingPropertyException:没有此类属性:$BUILD for class:groovy.lang.Binding

谢谢您!