Fix for checking-out voltha-system-tests repo patches

Change-Id: I79786c8d1cf4e549640b64d9152df328f7e182c0
diff --git a/jjb/pipeline/voltha-nightly-tests-bbsim.groovy b/jjb/pipeline/voltha-nightly-tests-bbsim.groovy
index 7370d07..5fb1278 100644
--- a/jjb/pipeline/voltha-nightly-tests-bbsim.groovy
+++ b/jjb/pipeline/voltha-nightly-tests-bbsim.groovy
@@ -84,6 +84,14 @@
             [$class: 'CloneOption', depth: 0, noTags: false, reference: '', shallow: false],
           ],
         ])
+        script {
+          sh(script:"""
+            if [ '${volthaSystemTestsChange}' != '' ] ; then
+              cd $WORKSPACE/voltha-system-tests;
+              git fetch https://gerrit.opencord.org/voltha-system-tests ${volthaSystemTestsChange} && git checkout FETCH_HEAD
+            fi
+            """)
+        }
       }
     }