generator-jhipster: Yarn not found in generated Jenkins pipeline file

Overview of the issue

I have used the latest JHipster 4 generator to create a simple (monolithic) app, selecting angular 2 and gradle as build system. Although JHipster is really awesome I get the following error when trying to run the generated build pipeline:

:npmInstall
> hippo-4@0.0.0 postinstall /var/jenkins_home/workspace/test-pipeline 
> yarn run webpack:build
sh: 1: yarn: not found

Motivation for or Use Case

I am running jenkins 2.32, if a Jenkinsfile is generated it should work out of the box.

Reproduce the error

To reproduce you should use latests release JHipster version, create a sample project using gradle as the build tool and angular2 lib (all other options can be default). Now try to use the generated Jenkinsfile on a jenkins server where no yarn is installed, the build should fail.

Suggest a Fix

JHipster Version(s)

4.0.2

JHipster configuration

{
  "generator-jhipster": {
    "jhipsterVersion": "4.0.2",
    "baseName": "hippo4",
    "packageName": "de.joergherbst.hippo",
    "packageFolder": "de/joergherbst/hippo",
    "serverPort": "8080",
    "authenticationType": "session",
    "hibernateCache": "ehcache",
    "clusteredHttpSession": false,
    "websocket": false,
    "databaseType": "sql",
    "devDatabaseType": "h2Disk",
    "prodDatabaseType": "postgresql",
    "searchEngine": false,
    "messageBroker": false,
    "buildTool": "gradle",
    "enableSocialSignIn": false,
    "rememberMeKey": "929ff6b80a3a4d1dbc6da49c965aeda3bd7ed2eb",
    "useSass": false,
    "clientPackageManager": "yarn",
    "applicationType": "monolith",
    "clientFramework": "angular2",
    "testFrameworks": [
      "gatling"
    ],
    "jhiPrefix": "jhi",
    "enableTranslation": true,
    "nativeLanguage": "en",
    "languages": [
      "en",
      "de"
    ]
  }
}

Entity configuration(s) entityName.json files generated in the .jhipster directory

no entity configured

Browsers and Operating System

Cause this is not an UI issue there is no browser dependency.