Install tinytex on appveyor for R

This took me a long time to figure out.

Points to note:

1. By default, appveyor.yml scripts don’t understand double quote marks. So start everything with cmd: and it will be fine.
2. Appveyor has a version of xz that doesn’t play nicely with tlmgr. So set the enviroment variable TEXLIVE_PREFER_OWN to 1, so tlmgr will use its own built-in copy of xz.

Here’s the relevant chunks of appveyor.yml, showing how to get tinytex installed and download a package. This assumes that tinytex is a dependency listed in your DESCRIPTION file.

environment:
  ...
  TEXLIVE_PREFER_OWN: 1
...
build_script:
  - travis-tool.sh install_deps
  - cmd: Rscript -e "tinytex::install_tinytex()"
  - cmd: Rscript -e "tinytex::tlmgr_install('latexdiff')"
  - cmd: Rscript -e "tinytex::tlmgr_path('add')"