Add timestamp to build-log.txt

[ST-DDT]

Based on https://forums.spongepowered.org/t/proofreaders-here-is-what-youre-waiting-for/23151/5

A while ago the build-log feature was added and made finding and fixing translation errors much easier.

For example (German), https://docs.spongepowered.org/stable/de/build-log.txt

A week (or so) ago I fixed (or at least tried to fix) the issues in that list, then a few days back I checked the list again and the list was the same. So I came up with the following proposal:

Can we alter this logic a little bit and add a timestamp to the top of the file, so that we can check whether there was a build after our last attempt of fixing that list?