Fix paths rbc-run uses to access the rbc tools.

This was broken in aosp/1878498; ccross@ submitted aosp/1875758 between
writing that change and submitting it, which changed the paths to mk2rbc
and rbcrun, thereby breaking Starlark product configuration.

Test: Presubmits.
Change-Id: I185489589997e6bdc815a54a8f6f328c18f813d0