Fix support for legacy schedule in build_from_flow

[bunchesofdonald]

The recent changes for enhanced scheduling broke the way the legacy schedule parameter worked for build_from_flow. This ensures that the legacy field is able to add and remove schedules, but in the case that both schedule and schedules are provided schedules will be the source of truth.

Closes #12255

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[abrookins]

Looks good 👍