They are already not reproducible.
But build time is not included, it's the commit time of the commit the build comes from, from what I gather.
Build time injected into builds would absolutely break any reproducibility.
What do you mean? Under what circumstances is a Go build not reproducible? From what I’ve observed, if you build your binary with a set of input, the same binary is produced. Is it sometimes not true?
Compiler version must be identical (not a huge stretch).
cgo reproducibility is not a guarantee.