Write variant environment variables to build.env

In this discussion on GTD we found that the environment variables which are automatically passed to all jobs in the pipeline are only those defined in the include files and at the top-level in .gitlab-ci.yml. If a variable value is modifed in a Composer variant job then this modified value is not propagated to the other jobs using that Composer variant.

This makes those variables inconsistent and therefore unusable in subsequent jobs. In particular, _LENIENT_ALLOW_LIST is not available for use in the PHPUnit job, to make dependent modules compatible for installing.

The variables that are set in the Composer variants in the templates are:

     DRUPAL_CORE
     PHP_VERSION
     IGNORE_PROJECT_DRUPAL_CORE_VERSION
     PHP_IMAGE_VARIANT
     _NODE_VERSION
     MINIMUM_STABILITY_OVERRIDE

Of these, DRUPAL_CORE, PHP_VERSION and PHP_IMAGE_VARIANT are already being written to build.env.

Proposed solution In addition to _LENIENT_ALLOW_LIST, also write IGNORE_PROJECT_DRUPAL_CORE_VERSION, _NODE_VERSION and MINIMUM_STABILITY_OVERRIDE to the build.env file in the Composer job.