I have asked about Julia's reproducibility story on the Guix mailing list in the past, and at the time Simon Tournier didn't think it was promising. I seem to recall Julia itself didnt have a reproducible build. All I know now is that github issue is still not closed.
"reproducible build" in this sense has nothing to do with scientific reproducibility. That issue is about hash-verifiability for the sake of security, and how some autogenerated random paths included in the binary affects that.
Scientific reproducibility requires only that versioned binaries be functionally equivalent if they have the same version, which is quite independent of this and certainly exists in Julia.
Would love a link to the Guix mailing list discussion, if you can dig it up.
https://github.com/JuliaLang/julia/issues/34753