Fix gitlab-ci.yml file indenting

For similar issues: 🐛 Test GitLab CI issue Active

Does not entirely seem to be solved by indenting. In the above, the 2.1.x branch was fixed by the indenting, but the 2.0.x branch needed to manually set _TARGET_PHP_IMAGE_VARIANT: 'apache' for all tests.

This is actually a problem with variable precedence, filed on this issue 🐛 Unable to run "composer (previous major)"; no image Active

This will be fixed when upstream fixes the testing infrastructure, but it's probably also good to fix the indent.

Fixed upstream. Minor changes here.

Automatically closed - issue fixed for 2 weeks with no activity.