release: fix overriding UNAME_r

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.

Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit c511a5ab53ba335dc85f7ac93843872eb5763602)
(cherry picked from commit 6462113b7012025c2d88451c1795368ef0607deb)