Make pak upgrade option configurable

[Fuco1]

Currently, it seems renv calls pak with this code

  pak$pkg_install(
    pkg     = packages,
    lib     = lib,
    upgrade = TRUE
  )

I think having to always upgrade is a bit too "agressive". In some cases I would like to only add packages and not upgrade what is not necessary.

Would it be possible to expose this as an option, either to renv::install (maybe through ... or as some .parameter) or as option for config?

[hadley]

Part of #1210