Document internal variables and make them consistent using build.env

jonathan1055 → changed the visibility of the branch main to hidden.

Changed title and updated IS, as this issue now covers the CURL variable. Because it was all to do with build.env I added them in to this MR. But if you would prefer the two enhancements separated out, that's fine I can make a new MR for the curl variables.

Readly for initial review and feedback.

The curl change was prompted by the error in 🐛 #3480767 Causes break in behavior of tests configuration Active

On first inspection, the code looks good. I'm glad we are actually simplifying the process and that we don't need to keep calling the snippet to recalculate the variables.

Downstream pipelines: https://git.drupalcode.org/issue/gitlab_templates-3487169/-/pipelines/33...

I'm happy that both sets of variables are covered in this issue, it's also good to have two snippets writing to build.env to make sure that the env file is built correctly and the all needed variables are set.

Not setting RTBC as we'll want some tests on other contrib modules, but it's looking good so far.

Tested https://git.drupalcode.org/project/scheduler/-/pipelines/342184
In addition to using this MR289 the Scheduler MR also:

• removes _TARGET_PHP_IMAGE_VARIANT: "apache" from a custom Drupal 9 job, and the tests still pass showing that this variable is propagated via writing to build.env.
• makes a change to a test file, to demonstrate that the CURL variables are being propagated as inrtended for test-only changes.

I will also review the doc pages as I expect there will be a few small changes relating to this.

I made a change to add an example of setting _TARGET_PHP_IMAGE_VARIANT: "apache" on docs/info/common.md#test-different-sqlite-versions

I don't have anything else to add here, so this is ready for review.

Downstream pipelines: https://git.drupalcode.org/issue/gitlab_templates-3487169/-/pipelines/34...

I was about to set it to RTBC in my last comment, so after the successful test in #7 plus the pipelines working, I can set it this time. Thanks!

Merged, thanks!