15.0.10: `slackpkg.conf` upgrade check always false

[opty77]

slackpkg/files/slackpkg

Line 74 in a64693d

WORKDIR=${ROOT}/${WORKDIR}

evaluating to / (at least) causes slackpkg.conf upgrade check

slackpkg/files/core-functions.sh

Line 207 in a64693d

if [ "${WORKDIR}" = "" ]; then

always false.