Set IMAGE and CURL variables for all subsequent jobs

Migrated issue

Reported by: jonathan1055

Related to !289 (merged)

Problem/Motivation

Some variables can be thoiught of "internal", either defined as global variables or derived in the composer jobs. They do not feature in the UI form because they vary between core versions. Not all of these are documented. Also some are not propagated to the subsequent jobs, which a customized value in Composer has to be repeated in PHPUnit and Nightwatch custom jobs.

Steps to reproduce

See https://app.slack.com/client/T06GX3JTS/CGKLP028K

Proposed resolution

• Write $PHP_IMAGE_VARIANT and $PHP_IMAGE_TAG to the build.env file in the composer job
• Write _CURL_TEMPLATES_REPO and _CURL_TEMPLATES_REF to build.env
• Remove *calculate-gitlab-ref in any job that 'needs' a composer job.
• Review https://project.pages.drupalcode.org/gitlab_templates/info/variants/#create-new-variants

Related issue: Issue #3486466