Vignette: Neater way of passing R_LIBS_USER on MS Windows

[HenrikBengtsson]

The vignette suggests to use:

BiocManager/vignettes/BiocManager.Rmd

Line 248 in ff48728

- Windows: `cmd /C "set R_LIBS_USER=C:\Users\USER_NAME\Documents\R\3.5-Bioc-3.7 && R"`

to launch R with R_LIBS_USER set that way. Not sure if you've already explored it, but R on Windows actually supports the following:

R R_LIBS_USER="C:\Users\USER_NAME\Documents\R\3.5-Bioc-3.7"

This has the advantage of (a) not having to mention cmd /C and (b) less error-prone since the value is quoted, which is needed for users with spaces in their Windows username.

The downside is that Rscript does not support this ¯\_(ツ)_/¯ (HenrikBengtsson/Wishlist-for-R#110)