Fix configuration of breaks when using recent scales package.

[plietar]

The latest release of the scales package has added a pair of optional arguments, d_transform and d_inverse, in the middle of the parameter list for trans_new (see https://github.com/r-lib/scales/pull/341). As a consequence, this has shifted the position of the breaks parameter from 4th to 6th.

Because the bench_time_trans and bench_bytes_trans functions from this package were passing in the breaks object positionally, this change in the scales package means that the breaks object is now matched with the new d_transform formal parameter and it ends up being ignored.

This commit changes the arguments to trans_new from being positional to being named, which should be more robust to future changes to the function.