Change repo for R packages in r.sif

[espenhgn]

Fixes #104

Changes proposed in this pull request:

• This replaces CRAN as the default source for R packages with daily snapshots from https://packagemanager.rstudio.com (2023-02-16)
• much faster r.sif build time as linux binaries are available
• added PRSice_linux to r.sif
• rebuilt r.sif (md5sum e83ffdd2282e81935db4a3a2fb307dcf)

Before submitting

• I've read and followed all steps in the Making a pull request
  section of the CONTRIBUTING docs.
• I've updated or added any relevant docstrings following the syntax described in the
  Writing docstrings section of the CONTRIBUTING docs.
• If this PR adds a new feature, I've added tests that sufficiently cover my new functionality.