Update Jenkinsfile build agent to improve build stability

Reuse of label for buildenv-agent means that the pod may be shared
across different builds, which leads to errors in the build at times. To
improve this, each project should have it's own unique label

Jenkinsfile

@@ -3,7 +3,7 @@
 pipeline {
   agent {
     kubernetes {
-      label 'buildenv-agent'
+      label 'buildenv-agent-openvsx'
       yaml '''
       apiVersion: v1
       kind: Pod