fix directory directive in jenkinsfile

Jenkinsfile

@@ -25,7 +25,7 @@ parallel 'centos7': {
         }
 
         stage('Build macOS UI') {
-            dir('$WORKSPACE/macui') {
+            dir('macui') {
                 sh 'xcodebuild -scheme "ZeroTier One" -configuration Debug'
             }
         }