When the nodejs tool is installed by using the tools block then:

• the NODEJS_HOME environment variable is set
• the PATH environment variable is expanded (inject path to the Node.js directory which keeps binaries)

Unfortunately, the NODEJS_HOME environment variable is not a standard. Many people use the NODE_HOME environment variable instead (see Node.js installation configuration on w3big.com). We could add some workarounds like

pipeline {
    agent any
    stages {
        stage('Build') {
            steps {
                nodejs(nodeJSInstallationName: 'Node 6.x', configId: '<config-file-provider-id>') {
                    withEnv(["NODE_HOME=${env.NODEJS_HOME}"]) {
                      sh 'buildScriptDevelopedByOtherPeople.sh'
                    }
                }
            }
        }
    }
}

but they are very messy. It would be great if both variables could be set. We need some changes in the NodeJSInstallation (extend) and NodeJSConstants (add new constant) classes. There is no need to add the PATH+NODE_HOME environment variable because the PATH+NODEJS_HOME environment variable handles the injection to the PATH environment variable correctly.