revdep_check ignores libpath

[gaborcsardi]

From https://github.com/r-lib/rcmdcheck/issues/71:

It seems, revdep_check ignores argument libpath when set to a character, e.g. "C:/Users/username/rlibs". What happens is that it tries to write to the default library. So, I can only pass a different libpath if I set the respective option and let the argument libpath with its default value ( libpath = getOption("devtools.revdep.libpath")).