Improve CI build speed.

[NaridaL]

Currently spends 5 mins building some c++ sources, which is probably unnecessary.

[seyfer]

@NaridaL any ideas how to improve it?

[pioug]

I was looking at it while changing the CI to GitHub Actions. I think we can update the baseline of this https://github.com/nfroidure/gulp-iconfont/blob/master/tests/tests.mocha.js#L231 by using an updated version of ttfautohint. Then we can simply do apt-get install ttfautohint instead of compiling a specific version.

But I am not sure exactly what ttfautohint does and how to verify that the new baseline is correct.