`$log()` has strange `dir` default

[simonpcouch]

If I set a dir when I make the Task, I'd prefer that that's the directory that the log is written to rather than inspect_log_dir() when I call $log(). This would also make behavior consistent with $eval().

[simonpcouch]

The solution here is to locate where the dir argument is stored on $new():

vitals/R/task.R

Line 142 in 68a312f

dir = vitals_log_dir()

...and then make that the new default in $log():

vitals/R/task.R

Line 335 in 68a312f

log = function(dir = vitals_log_dir()) {

Note that $eval() also calls $log(), but it seems to use the right log directory.