Add CRAN-like mirror for R packages compiled for Wasm

[hturner]

Once webR is add to image (carpentriesoffline/carpentriesoffline-installer#43) we would want a CRAN-like mirror of R packages compiled for Wasm, to avoid needing a network connection to install packages.

Note that such packages can be obtained from https://ropensci.org/blog/2023/11/17/runiverse-wasm/.

Thanks to @georgestagg for the pointer.