release: More fixes for building pkg package

• Ignore "wrong major" errors when obtaining WRKDIR.
• Put distfiles in /tmp in case /usr/ports is read-only.

MFC after: 6 hours
Sponsored by: https://www.patreon.com/cperciva

(cherry picked from commit 088e21000507510a2c3b1ec22684fe9665f6c8a7)