zuul-var-to-json: parameterization of the target path

Change-Id: I7f67bfccffeed32ddfa3c7e033cb401c0806598c
diff --git a/roles/zuul-var-to-json/tasks/main.yaml b/roles/zuul-var-to-json/tasks/main.yaml
index 5d6e1d7..01c5da4 100644
--- a/roles/zuul-var-to-json/tasks/main.yaml
+++ b/roles/zuul-var-to-json/tasks/main.yaml
@@ -1,4 +1,4 @@
 - name: Export Zuul variables into JSON
   copy:
-    dest: '{{ ansible_user_dir }}/zuul-env.json'
+    dest: '{{ zuul_env_json_file }}'
     content: '{{ zuul | to_json }}'