We include glyphs in the font that fontmake excludes

Not totally sure what's up here; there is no `public.skipExportGlyphs` present, although there is the slightly mysterious (to me) key `com.schriftgestaltung.Glyphs.Export` in the glyph's lib. (this is also present in some other glyphs that _do_ seem to be exported, so I'd just call this a clue?)

```
python3 -m ttx_diff 'https://github.com/notofonts/chakma?239fe3aac7#sources/NotoSansChakma.designspace'
```

Start by running the provided ttx_diff command against the NotoSansChakma.designspace source and compare which glyphs fontc includes with those fontmake excludes. Inspect the glyph lib for com.schriftgestaltung.Glyphs.Export and related export metadata; done means identifying why the tools disagree and confirming the behavior against the source.

Written by the indexing model from the issue text.