BUG FIX: Invalid R_LIBS* paths on Windows

[HenrikBengtsson]

safe_install_packages() used a colon as a path separator also on Windows, resulting in invalid R_LIBS* values.

For example, with startup::startup() in ~/.Rprofile, which checks for this, using remotes::install_*() would produce warnings about this:

Warning messages:
1: Environment variable 'R_LIBS_SITE' specifies a non-existing folder 'C:/Users/hb/Documents/R/win-library/3.5:C:/Program Files/R/R-3.5.1/library' (expands to 'C:\Users\hb\Documents\R\win-library\3.5:C:\Program Files\R\R-3.5.1\library') which R ignores and therefore are not used in .libPaths(). To create this folder, call dir.create("C:/Users/hb/Documents/R/win-library/3.5:C:/Program Files/R/R-3.5.1/library", recursive = TRUE) 
2: Environment variable 'R_LIBS_USER' specifies a non-existing folder 'C:/Users/hb/Documents/R/win-library/3.5:C:/Program Files/R/R-3.5.1/library' (expands to 'C:\Users\hb\Documents\R\win-library\3.5:C:\Program Files\R\R-3.5.1\library') which R ignores and therefore are not used in .libPaths(). To create this folder, call dir.create("C:/Users/hb/Documents/R/win-library/3.5:C:/Program Files/R/R-3.5.1/library", recursive = TRUE) 

[gaborcsardi]

Thanks!