Splitting pattern for RENV_CONFIG_EXTERNAL_LIBRARIES does not work well on Windows.

[MilesMcBain]

When using env var RENV_CONFIG_EXTERNAL_LIBRARIES the code here:

https://github.com/rstudio/renv/blob/281ddcb90b2f742ac9e6e15682f44a9ad9858dd8/R/config.R#L137C5-L137C20

selects string splitting pattern "\\s*[:;,]\\s*" which splits a Windows path like C:/Users/username/r_library/ into two pieces due to the comma. It seems that I get lucky and that /Users/username/r_library/ is defaulted to C: drive somewhere along the way. But as an unfortunate side effect a new empty folder called "C" (corresponding to the first split) is created in my project root.

[kevinushey]

Thanks for the bug report -- I'll take a look.

[kevinushey]

I pushed a potential bandaid for this in d6fe08d -- are you able to confirm if that fixes the issue on your side as well?