gapic-generator-java embeds timestmaps in output

[benjaminp]

gapic-generator-java creates JarEntrys without setting a timestamp on them, JarOutputStream.putNextEntry helpfully sets the timestamp to System.currentTimeMillis() in that case breaking determinism of the generator.

[blakeli0]

Thanks @benjaminp for creating the issue! Do you have a specific use case for this issue? I think it's nice to have but may not be needed.

[benjaminp]

rules_gapic allows one to hook up the gapic generator into a Bazel build, where reproducibility of actions is important to maximize caching hit rates as well as the other benefits listed on https://reproducible-builds.org/. This report arose from an internal build indeterminism detector that I have that runs build actions over and over to see if they produce the same result.