[r-apt] Option to setup `littler`'s install scripts

[eitsupi]

It is tedious to write R functions in options such as postCreateCommand of the json file because double quotes need to be escaped.

For r-rig, I assume that it is recommended to use pak or renv, but for r-apt (especially if bspm is enabled on Ubuntu) it is preferable to use the install.packages function, so scripts of littler may be useful.