Adding `environment` keyword to pages jobs in CI/CD templates

What does this MR do and why?

The purpose of this MR to resolve #370964 (closed)

This adds the environment keyword to the pages job in the following CI/CD template files:

• https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/ci/templates/Pages/Lektor.gitlab-ci.yml
• https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/ci/templates/Pages/Gatsby.gitlab-ci.yml
• https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/ci/templates/Pages/Jekyll.gitlab-ci.yml
• https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/ci/templates/Pages/Octopress.gitlab-ci.yml
• https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/ci/templates/Pages/Nanoc.gitlab-ci.yml