Original variables when running a manual job are not reused on job retry

Problem

Implemented in gitlab-foss#24935 (closed) is the ability in the UI to specify variables/job parameters when running a manual job, however when retrying a failed manual job the previously specified parameters fail to be reused.

Proposal

When a manual job is retried, it should inherit the variables that were specified for its run.

Further details

In next iterations we'd like to surface the existing manual job variables in the Job page UI and make it possible to edit them to make it easier to use variables with manual jobs like deploy jobs.