Write core version variables to build.env in D7 pipelines

Whilst working on this GTD issue I found that in the D7 pipeline neither of the variables $DRUPAL_CORE nor $INSTALLED_DRUPAL_VERSION are written to build.env. These are written in the modern version pipeline.

This omission makes it harder to write some common config before_script_actions where nearly everything is the same and the version can be used to make it completely generic. See https://git.drupalcode.org/issue/gitlab_templates_downstream-3573349/-/jobs/9047593

I suggest that we write both of these variables to build.env for consistency and to help with that issue