Extract multi-platform Tekton Pipeline

Actions taken:
* Move the PipelineSpec content into a Pipeline file
* Rearranged the spec to be ordered params, tasks, results, finally,
  workspaces
* Added back the on-cel-expressions from before (changing the pipeline
  which will trigger builds from multi-arch-build-pipeline)
* Added spec.params.build-platforms to the PipelineRun
* Changed the build-image-index task to accept a single value for IMAGES
  to allow for a single arch build for initial builds since
  tektoncd/pipeline#8158 is not deployed to the
  Konflux cluster yet.

Signed-off-by: arewm <[email protected]>