Ignore build folder for CSS linting (#4264)

Spotted in https://circleci.com/gh/wagtail/wagtail/1512.

.stylelintrc.yaml

@@ -1,5 +1,6 @@
 ignoreFiles:
   - node_modules
+  - build/**/*  
 plugins:
   - stylelint-scss
 # See https://github.com/stylelint/stylelint/blob/master/docs/user-guide/rules.md