GenerateNLVersion.kt prevents reproducible builds for NL variants

[gagath]

The scripts/GenerateNLVersion.kt program generates intermediate files by creating .ttx files, modifying them, and converting them back to .ttf files. When doing so, the modification date of the head table for NL fonts is never the same and depends on the date the ttx file where converted back into ttf files. This is visible in the following diffoscope output:

This is preventing reproducible builds of the NL variants of the font.