Dude, Debian does exactly this same exact thing to get reproducible builds. They generally submit things upstream, so I'm not aware of Nix attempts do the same, but I can also imagine their patches for reproducible builds might not make sense outside of Nix. This particular patch is done differently than how Debian attempts to do it (they try to make no changes to the code if possible), but this patch doesn't seem like a bad approach either (makes it obvious at least).