gftools-rename-font does not rename instance PS names

`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 ""...
Dumping 'name' table...
PlaypenSans-Thin
PlaypenSans-ExtraLight
PlaypenSans-Light
PlaypenSans-Regular
PlaypenSans-Medium
PlaypenSans-SemiBold
PlaypenSans-Bold
PlaypenSans-ExtraBold
```