Mk/Uses/npm.mk: Quote TMPDIR and WRKDIR in shell comparisons

When TMPDIR is unset, the generated shell command becomes:

if [ != ${WRKDIR} ]; then

and /bin/sh reports:

[: !=: unexpected operator

Quoting both variables fixes the issue.

PR: 297181
Reported by: Shin-ichi Nagamura <core@zsc.jp>

(cherry picked from commit cbeb1be34ba3107114345e52c6b57adc2ec34060)