feat(web-ext-lint): allow using project's web-ext package

Change-Id: I74889983796e96754e0c9347f5b94d6a7da02c76
diff --git a/zuul.d/jobs.yaml b/zuul.d/jobs.yaml
index e60881d..ffe92c8 100644
--- a/zuul.d/jobs.yaml
+++ b/zuul.d/jobs.yaml
@@ -3,6 +3,8 @@
     pre-run: playbooks/web-ext-lint/pre.yaml
     run: playbooks/web-ext-lint/run.yaml
     post-run: playbooks/web-ext-lint/post.yaml
+    vars:
+      use_standalone_web_ext: true
     description: |
       Check a Web Extension project for basic lint problems.