Use install script from r-appveyor to handle different download URLs

[nuest]

this might simplify the Dockerfile:

https://github.com/krlmlr/r-appveyor/blob/master/scripts/appveyor-tool.ps1#L44-L112

thanks @jeroen for the pointer!

[nuest]

On the negative side, this reduces the stand-alone-ness of the Dockerfile.