Some other improvements to installation logfiles

[njtierney]

• Consider having both environment variable and logfile(?) argument in install_greta_deps()
• tools::R_user_dir() could be used to always write a logfile to a file, rather than needing the user to specify it

(Also from @maelle :) )

[maelle]

plugging a blog post :smile: https://blog.r-hub.io/2020/03/12/user-preferences/