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

Change-Id: I74889983796e96754e0c9347f5b94d6a7da02c76
diff --git a/playbooks/web-ext-lint/pre.yaml b/playbooks/web-ext-lint/pre.yaml
index ccd2fad..37d845b 100644
--- a/playbooks/web-ext-lint/pre.yaml
+++ b/playbooks/web-ext-lint/pre.yaml
@@ -1,4 +1,6 @@
-- name: Ensure web-ext is installed
-  hosts: all
-  roles:
-    - ensure-web-ext
+- hosts: all
+  tasks:
+    - name: Ensure web-ext is installed
+      include_role:
+        name: ensure-web-ext
+      when: use_standalone_web_ext