Return correct path from check_r_cmd()

Previously, `check_r_cmd()` always returned a string indicating that the
check output was built in `tempdir()`, even when `check_dir` was
custom-set by the user.  Now the returned string indicates the correct
location in which the check output was built.
Add attribution for bug fix
Merge branch 'bugfix/print-check-dir' of https://github.com/brentonk/devtools into brentonk-bugfix/print-check-dir