use gha instead of travis

[cderv]

Customizing actions from r-lib/actions one.

Unfortunately, not sure we can cache CTAN packages. One way to deal with that would be to use a set of docker images with with pre-installed component.

We'll how long it takes with this config

[cderv]

@yihui just a question about ctex and Chinese fonts.

Currently it errors on Windows and Mac because fonts are not found:

• Mac - Kaiti SC: https://github.com/rstudio/rticles/runs/1391411927?check_suite_focus=true#step:14:139
• Windows - SimHei: https://github.com/rstudio/rticles/runs/1391411968?check_suite_focus=true#step:14:221

Do you know if these fonts can be added easily on those OS by command line ? If not, don't bother looking - I'll just skip this test of ctex format on Windows and Mac to just run it on Linux.

Thank you.

[cderv]

I am merging this, but if you have the trick for the fonts, feel free to tell me and I'll add it.

Thanks

[yihui]

I know how to make it work on macOS and Windows (we could use https://ctan.org/pkg/fandol), but I think it's okay for the ctex test to be run only on Linux. Thanks!

[cderv]

I leave it that way then