Fix regression in bsdinstall post rS356740 - partedit errno(2) 21 EISDIR
This resulted in the partitioning step failing if either of the
"Auto (UFS)" or "Manual" options were selected, with error:
Abort ----- Partioning error An installation step has been aborted. Would you like to restart the installation or exit the installer?
partedit was attempting to open a directory (TMPDIR) read/write,
which resulted in errno(2) 21 - EISDIR - Is a directory.
# /usr/libexec/bsdinstall/partedit partedit: /tmp: Is a directory