gftools-rename-font does not rename instance PS names

[simoncozens]

gftools-rename-font (which uses the axisregistry build_name_table function to do its work) claims to change "the PostScript name" - which it does, partially. However, any fvar table instance PS names are not renamed:

$ gftools-rename-font --just-family -o PPS-Deva.ttf fonts-builder/variable/PlaypenSans\[wght\].ttf "Playpen Sans Deva" --just-family
Saving font: PPS-Deva.ttf
$ ttx -o - -t name PPS-Deva.ttf | grep 'PlaypenSans-'
Dumping "PPS-Deva.ttf" to "<stdout>"...
Dumping 'name' table...
      PlaypenSans-Thin
      PlaypenSans-ExtraLight
      PlaypenSans-Light
      PlaypenSans-Regular
      PlaypenSans-Medium
      PlaypenSans-SemiBold
      PlaypenSans-Bold
      PlaypenSans-ExtraBold