runtime/qio: use getcwd instead of getwd

getwd is deprecated in favor of getcwd.

Related to #325.
Is the directory perhaps not getting created here?  My coworker said we might not be able to test this case because the system might not allow us to create a directory with that long a path name.  The preexec will not necessarily put its error messages into the tmp file, so it is easy to miss things that failed in there (we're working on our test system gradually).