Switch over to Build associations for DAST On-demand CI Variable Delivery

The following discussion from !63362 (merged) should be addressed:

• @philipcunningham started a discussion:

  backend

  i expect that we will want to use this variable association mechanism over the existing one (via ci_pipelines association) but keep the notion of there being a trigger for on-demand. i would like to do this as a follow-up in a subsequent milestone. does that work for you?