Jenkinsfile: set USER=root to docker run

This to fix build failure appeared in CI after recent govm update:

-----
  make start
  tar zxf _work/govm_0.9-alpha_Linux_amd64.tar.gz -C _work/bin/
  PATH="/mnt/workspace/pmem-csi_PR-461/_work/bin:$PATH" test/start-kubernetes.sh
  get current user: user: Current requires cgo or $USER set in environment
  make: *** [test/start-stop.make:13: start] Error 1
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Author: avalluri

Jenkinsfile

@@ -354,6 +354,7 @@ String DockerBuildArgs() {
     -e BUILD_IMAGE_ID=${env.CACHEBUST} \
     -e 'BUILD_ARGS=--cache-from ${env.BUILD_IMAGE} --cache-from ${env.PMEM_CSI_IMAGE}' \
     -e REGISTRY_NAME=${env.REGISTRY_NAME} \
+    -e USER=root \    
     -v /var/run/docker.sock:/var/run/docker.sock \
     -v /usr/bin/docker:/usr/bin/docker \
     -v `pwd`:${env.PMEM_PATH} \