Makefile: Use the same modification date for all fonts

Gioelerequested to merge
gioele/xolonium:same-modtime into master

Currently all modification dates in the fonts are a few seconds different from each other, due to the time spent actually creating the font. This makes reproducing the released font files a bit more difficult because there are different timestamps to reproduce.

To avoid this issue, now that make-font.py supports SOURCE_DATE_EPOCH, let's fix the modification time for all fonts at once.

Merge request reports