This aims to bring more consistency with the rest of core, and more closely mirrors the similar changes upstream to use `sys_get_temp_dir()`.
The potential `false` return value was not checked by the only caller in `Text_Diff_Engine_shell::diff()`.
Follow-up to [7747], [48464], [49185], [60776].
Props TimoTijhof, apermo, SergeyBiryukov.
Fixes#63711.
Built from https://develop.svn.wordpress.org/trunk@60777
git-svn-id: http://core.svn.wordpress.org/trunk@60113 1a063a9b-81f0-0310-95a4-ce76da25c4cd