Write variant environment variables to build.env
In [this discussion on GTD](https://git.drupalcode.org/project/gitlab_templates_downstream/-/issues/3571809#note_710186) 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.
issue