Jenkinsfile: Re-use global VersionNumber for installers

Commit 3611bdcb moved the VersionNumber generation at the beginning of the
pipeline so we only generate it once.

The installer merge landed at roughly the same time so it missed that.

Align the stage generating the installer images to the new approach.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

Jenkinsfile

@@ -409,8 +409,7 @@ def buildInstallers(installerTargets) {
 
             buildenv.inside("--device=/dev/kvm") {
               stage("setup installers") {
-                env.PIPELINE_VERSION = VersionNumber(versionNumberString: '${BUILD_DATE_FORMATTED,"yyyyMMdd"}.${BUILDS_TODAY_Z}')
-                  sh ("env ; mkdir -p ${PIPELINE_VERSION}/installer")
+                sh ("env ; mkdir -p ${PIPELINE_VERSION}/installer")
               }
 
               def buildStatus = [:]