Validatable config job is broken when MR introduces new dependencies

Problem/Motivation

The validatable config job does a git checkout at the tip of an MR, but then immediately checks out HEAD before running composer commands. This means it misses out on new dependencies introduced in the MR.

Sidenote: we really need a "CI" or "Gitlab" component for d.o issues.

Steps to reproduce

See https://git.drupalcode.org/issue/drupal-3458945/-/jobs/2307318

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet