refactor: add web-ext dependency and use it in the lint check

This commit makes the lint pipeline run web-ext from this repo's
package.json dependency, which will be cached in pnpm's store and thus
be installed quicker than if it was installed standalone.

Bug: twpowertools:212
Change-Id: I6da60ac9b858b8298cbc992ebf208da1d5b00094
diff --git a/.zuul.yaml b/.zuul.yaml
index 10fd574..ed7cd8e 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -12,6 +12,9 @@
     pre-run: playbooks/lint/pre.yaml
     run: playbooks/lint/run.yaml
     post-run: playbooks/lint/post.yaml
+    vars:
+      use_standalone_web_ext: false
+      web_ext_command: "./node_modules/web-ext/bin/web-ext.js"
     irrelevant-files:
       - ".github/.*"
       - "docs/.*"