Fix LIBPATH on Jenkins pipeline on z/OS

Currently due to a limitation of the z/OS CMake port we require to
specify an explicit `LIBPATH` so that the various tests which
dynamically load DLLs can find them. Once CMake is fixed to properly
generate the dynamic libraries in the correct directories setting of
the `LIBPATH` environment variable should be removed. See eclipse-omr#4761.

Signed-off-by: Filip Jeremic <fjeremic@ca.ibm.com>

buildenv/jenkins/jobs/builds/Build-zos_390-64

@@ -13,7 +13,7 @@ def setBuildStatus(String message, String state, String sha) {
 pipeline {
     agent{label 'zOS && 390'}
     environment {
-        LIBPATH=".:$LIBPATH"
+        LIBPATH="/openzdk/jenkins/workspace/Build-zos_390-64/build:$LIBPATH"
     }
     stages {
         stage('Get Sources') {

buildenv/jenkins/jobs/pull-requests/PullRequest-zos_390-64

@@ -1,7 +1,7 @@
 pipeline {
     agent{label 'zOS && 390'}
     environment {
-        LIBPATH=".:$LIBPATH"
+        LIBPATH="/openzdk/jenkins/workspace/PullRequest-zos_390-64/build:$LIBPATH"
     }
     stages {
         stage('Get Sources') {