For steps that have a single input that never changes when re-running make.
pub fn singleUnchangingWatchInput(step: *Step, maker: *Maker, arena: Allocator, lazy_path: LazyPath) Allocator.Error!void
pub fn singleUnchangingWatchInput(step: *Step, maker: *Maker, arena: Allocator, lazy_path: LazyPath) Allocator.Error!void {
if (!step.inputs.populated()) try step.addWatchInput(maker, arena, lazy_path);
}