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

Change-Id: I74889983796e96754e0c9347f5b94d6a7da02c76
diff --git a/roles/web-ext-lint/tasks/main.yaml b/roles/web-ext-lint/tasks/main.yaml
index 57b5b12..0ce8fcb 100644
--- a/roles/web-ext-lint/tasks/main.yaml
+++ b/roles/web-ext-lint/tasks/main.yaml
@@ -3,7 +3,8 @@
     cmd: |
       set -o pipefail
       set -e
-      web-ext lint --no-input --boring \
+      {{ 'web-ext' if use_standalone_web_ext else web_ext_command }} \
+          lint --no-input --boring \
           {{ '--source-dir=' + source_dir if source_dir else '' }} \
           {{ extra_lint_flags }} | tee -a web-ext-lint.txt
     chdir: "{{ zuul.project.src_dir }}"