Store the STAGE_NAME in a variable and read it in the post-section

examples/jenkins/report_stage.jenkinsfile

pipeline {
    agent { label 'master' }
    stages {
        stage('first') {
            steps {
                script {
                    last_started = env.STAGE_NAME
                    echo "in first"
                    def a = 0
                    def b = 10 / a
                }
            }
        }
        stage('second') {
            steps {
                script {
                    last_started = env.STAGE_NAME
                    echo "in second"
                }
            }
        }
    }
    post {
        failure {
            echo "last_started: $last_started"
        }
    }
}