pager: set $LESSSECURE whenver we invoke a pager

pager: set $LESSSECURE whenver we invoke a pager

Some extra safety when invoked via "sudo". With this we address a
genuine design flaw of sudo, and we shouldn't need to deal with this.
But it's still a good idea to disable this surface given how exotic it
is.

Prompted by #5666

(cherry picked from commit 612ebf6c913dd0e4197c44909cb3157f5c51a2f0)