Hi, Rich. It does this:
#!/bin/sh
fc-cache -r
It wreaks havoc during boot (delay and several errors/warnings). My workaround is to just delete the script from the extension. There are no bad side effects as far as I can tell.
fontconfig.tcz is a dependency of Hack-font.tcz and is therefore also being loaded at boot time. Since tce.installed scripts are run as a batch after all extensions have loaded, by the time tce.installed/fontconfig runs, it "sees" Hack-font and includes it in the font cache.
I guess the only time Hack-font's tce.installed script would be needed is when the extension is loaded after boot.