Cater for node version in manual UI pipelines

Migrated issue

Reported by: fjgarlin

Problem/Motivation

#3561429: Support custom node versions on testing added the option to change the node version shipped with the image via "nvm".
#3563698: Drupal 11.3 and 10.6 are released - update variant variables used that feature as "previous major" core (10.6.x at the time) required v20.

That is working well, but when pipelines are triggered via the web UI, "previous major" composer fails as the variable is shown in the web UI form and the default is empty. This has priority over the per-job override.

Steps to reproduce

❌ https://git.drupalcode.org/project/layout_builder_st/-/jobs/7908094 (triggered via web UI)
✅ https://git.drupalcode.org/project/layout_builder_st/-/jobs/7897305 (triggered via MR)

Proposed resolution

Try to set the right default out of the box instead of leaving it empty.