release.sh: fix OSVERSION and UNAME_r overrides

Add PBUILD_FLAGS and UNAME_r overrides to extra_chroot_setup()
to fix building ports for 14-CURRENT builds on 13.x.

MFC after: 3 days
MFC with: c511a5ab53ba335dc85f7ac93843872eb5763602
Sponsored by: Rubicon Communications, LLC ("Netgate")