devel/root: Fix building on 12.3, 13.2 and restore post-patch step

ROOT version 6.28.02 fails to build on 123amd64:
https://pkg-status.freebsd.org/beefy6/data/123amd64-default/d55ec1db8ffa/logs/root-6.28.02.log
This patch fixes the (sed-related) issue.

I submit here - again - the post-patch step, which ensures that ROOT can
build on LLVM/Clang 10.0.1 (used on FreeBSD 12.3).

Fix plist on 13.2 (RC)

PR: 270625