Remove some non-determinism

Your app is not built reproducible in F-Droid, but we continuously test older versions on https://verification.f-droid.org/ and would like more and more apps to become repro.

Looking at this report: https://verification.f-droid.org/com.superproductivity.superproductivity_1200000000.apk.diffoscope.html#assets-public-ngsw.json

Can you maybe set timestamp to be based on the git tree last commit date like git log -1 --pretty=%ct ?