Fix "pages not in nav" command

What does this MR do and why?

The current invocation of make check-pages-not-in-nav fails because we also run install-nodejs-dependencies which has some Yarn output that jq doesn't like. It seems this behavior changed when we updated to Yarn 4.

Additionally, an exclusion for /user/application_security/dast/browser/checks/ has been added to the script to exclude those pages from the check. Seems the old location was changed.

Reported in technical-writing#985 (comment 1778462638)

How to set up and validate locally

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.
2. Run the following and make sure there's no error:

   make check-pages-not-in-nav
   make check-pages-not-in-nav | jq '.[] | .url'

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