Fixes "retryable" value for merge train pipelines as JSON

What does this MR do and why?

Implements #436348 (closed), part of #442453 (closed)

This change updates the value of "retryable" in merge train pipelines when the pipeline is fetched with JSON format.

This allows the "retryable" to stay consistent across the GraphQL API and other models.

Changelog: fixed

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Before	After

How to set up and validate locally

1. You'll need a .gitlab-ci.yml compatible with merge trains, that also happens to pass during a regular pipeline and fail during a merge train.

I used the following:

workflow:
  rules:
    - if: $CI_PIPELINE_SOURCE == 'merge_request_event'

stages:
  - build

normal-job:
  stage: build
  environment: production
  script:
    - echo "Running..."
    - sleep 20
    - echo "Done."

fail-on-merge-train:
  stage: build
  rules:
    - if: $CI_MERGE_REQUEST_EVENT_TYPE == 'merge_train'
  script:
    - echo $CI_MERGE_REQUEST_EVENT_TYPE
    - echo "Failing because this is a merge train, sorry!"
    - exit 1

2. Enable merge trains by selecting "Enable merge trains" at Project -> Settings -> Merge requests (e.g. http://gdk.test:3000/my-group/my-merge-trains/-/settings/merge_requests)
3. Open an MR with an arbitrary change
4. Hit Merge and wait for the merge train pipeline to fail
5. Visit the Pipelines tab and confirm there is no retry button (e.g. http://gdk.test:3000/my-group/my-merge-trains/-/merge_requests/5/pipelines)