Reproducible builds are highly important indeed, though from security perspective we shouldn't assume we're secure because we can reproduce the build.
As Ken Thompson shows us in Reflections on Trusting Trust: backdoors can live in binary form only and the source code may not be telling the full story.
What we need to trust are the entities writing and distributing the packages, of everything. Sadly today there is no way to answer: what is the set of entities I'm trusting by using this package.