fix: the "Push Docker Image" jenkins step requires a HOME var (#2748)

The requirement is by top-level code in "bash_standard_lib.sh"
(provided by infra) and used in "push-docker.sh" when run in CI.

Author: trentm

.ci/Jenkinsfile

@@ -6,6 +6,7 @@ pipeline {
   environment {
     REPO = 'apm-agent-nodejs'
     BASE_DIR = "src/github.com/elastic/${env.REPO}"
+    HOME = "${env.WORKSPACE}" // required by "bash_standard_lib.sh" for "Push Docker Image" step
     PIPELINE_LOG_LEVEL='INFO'
     JOB_GCS_BUCKET = credentials('gcs-bucket')
     GITHUB_CHECK_ITS_NAME = 'Integration Tests'