[D7] Composer 1 only needed if _D7_DRUPAL_COMPOSER_NEEDED is true
Problem/Motivation
The composer step uses composer 1. This was due to "drupal/composer" package, but bringing this package is now behind a variable, so we might be able to speed up other runs if we only switch to composer 1 when _D7_DRUPAL_COMPOSER_NEEDED is set to 1, otherwise default to composer 2.
Steps to reproduce
Run any D7 contrib pipeline
Proposed resolution
Put the comkposer self-update --1 behind a conditional checking _D7_DRUPAL_COMPOSER_NEEDED is 1.
7.x modules used here for gitlab_templates MR testing
packages.drupal.org (aka project_composer)
This has _TARGET_PHP: "5.6" so already using Composer 1 from the start
_D7_DRUPAL_COMPOSER_NEEDED is not specified so takes the default value 0
MR12 project_composer!12
API 7.x-2.x
Has _TARGET_PHP: "5.6" so already has Composer 1 at the start
Has _D7_DRUPAL_COMPOSER_NEEDED: "1"
MR22 api!22
Scheduler 7.x-1.x
No value of _TARGET_PHP so defaults to latest, and hence starts off with Composer 2
Has _D7_DRUPAL_COMPOSER_NEEDED: "1"
MR99 scheduler!99 (closed)
i18n module
This has completely un-customised .gitlab-ci.yml template, therefore uses Composer 2 and does not require _D7_DRUPAL_COMPOSER_NEEDED
MR6 i18n!6
The above four MRs can remain open for future testing.
Related issue: Issue #3408456