jenkins: fix user.name in docker

User 910 is unknown in the docker container, so 
`${id -un 910}` does not return a name. We don’t 
care what user.name it has, as long as there is one.
diff --git a/Jenkinsfile b/Jenkinsfile
index e06ad4b..f37b75f 100644
--- a/Jenkinsfile
+++ b/Jenkinsfile
@@ -39,7 +39,7 @@
 
             stage('Run tests') {
                 environmentDockerImage.inside('-i --rm --name brooklyn-${DOCKER_TAG} -u 910:910 --mount type=bind,source="${HOME}/.m2/settings.xml",target=/var/maven/.m2/settings.xml,readonly -v ${WORKSPACE}:/usr/build -w /usr/build') {
-                    sh 'mvn clean install -Duser.home=/var/maven -Duser.name=$(id -un 910)'
+                    sh 'mvn clean install -Duser.home=/var/maven -Duser.name=jenkins'
                 }
             }
 
@@ -47,7 +47,7 @@
             if (env.CHANGE_ID == null) {
                 stage('Deploy artifacts') {
                     environmentDockerImage.inside('-i --rm --name brooklyn-${DOCKER_TAG} -u 910:910 --mount type=bind,source="${HOME}/.m2/settings.xml",target=/var/maven/.m2/settings.xml,readonly -v ${WORKSPACE}:/usr/build -w /usr/build') {
-                        sh 'mvn deploy -DskipTests -Duser.home=/var/maven -Duser.name=$(id -un 910)'
+                        sh 'mvn deploy -DskipTests -Duser.home=/var/maven -Duser.name=jenkins'
                     }
                 }
             }