We now track copied files in /etc/.clean. This is important, because
otherwise files that are removed from environment.etc will not
actually be removed from the file system. In particular, changing
users.extraUsers.<user>.openssh.authorizedKeys.keys to an empty list
would not cause /etc/ssh/authorized_keys.d/<user> to be removed, which
was a security issue.
Using pkgs.lib on the spine of module evaluation is problematic
because the pkgs argument depends on the result of module
evaluation. To prevent an infinite recursion, pkgs and some of the
modules are evaluated twice, which is inefficient. Using ‘with lib’
prevents this problem.
Some programs (notably the Java Runtime Environment) expect to be able
to extract the name of the time zone from the target of the
/etc/localtime symlink. That doesn't work if /etc/localtime is a
symlink to /etc/static/localtime. So make it a direct symlink.
