Go builds are, in general, reproducible by default. Build stamps do not change this.
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.