mktemp: use map_err instead of match

[cakebaker]

No description provided.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/cut/cut-huge-range is now passing!
Congrats! The gnu test tests/printf/printf-surprise is now passing!
Congrats! The gnu test tests/tail/tail-n0f is now passing!