Do not render the home page in upstream products

The home page may contain relative links to one of the four products we build the docs from that may not be present when the site is built and tested in the upstream products.

For example, when the docs site is built and tested for dead relative links in the Runner project, it will fail for some relative links in the front page that point to the Omnibus docs, since the Omnibus docs are absent.

Follows up on !1707 (merged) and is another fix for gitlab#328196 (closed).

After this is merged we'll need to build the Docker image that the upstream products use for testing.

Test locally

Check out the branch:

git fetch origin
git checkout -b "ap-frontpage-skip-in-single-product-tests" "origin/ap-frontpage-skip-in-single-product-tests"

In the context of Omnibus, Runner and Charts, the GitLab docs are not present, so remove them before building the site:

rm -r content/ee

Test that without setting the variable, the front page is built and the internal check fails:

rm -rf public
bundle exec nanoc
bundle exec nanoc check internal_links

Test that setting the variable to gitlab-runner, the front page is not built, and the tests pass:

rm -rf public
CI_PROJECT_NAME=gitlab-runner bundle exec nanoc
bundle exec nanoc check internal_links