Resolved: How to append to a file in Jenkins pipeline?

node("jenkins-builder") {
    stage("test") {
        String s = "JIRA-1234 | Use "'" in place of apostrophes in patient's names when exporting to CSV."
        dir(env.WORKSPACE) {
            writeFile(file: "./out.txt", text: "First line")
            writeFile(file: "./out.txt", text: s)
            def f = readFile(file:"./out.txt")
            println f
        }
    }
}

def content = readFile(file:"out.txt")
content+="new line\n"
writeFile(file:"out.txt", content)

new File("${env.WORKSPACE}/out.txt").append("new line")