[Rd] Default for checkBuilt in update.packages() should be TRUE

[Duncan Murdoch]

The checkBuilt argument to update.packages() currently defaults to 
FALSE.  This means that packages built and installed under R 3.6.2 will 
not be updated by R 4.0.0, and leads to confusion (e.g. 
https://stackoverflow.com/q/60356442/2554330, where tidyverse can't be 
updated because some of its many dependencies haven't been updated).

The default should be TRUE, even though this will lead to some packages 
being updated unnecessarily, because the cost of an unnecessary update 
is so much less than the cost of missing a necessary update.

Duncan Murdoch