Fixed override of KRB5CCNAME when attaching.

Fixed override of KRB5CCNAME when attaching.

Because KRB5CCNAME was set in update-environment, when attaching
to a running session the saved KRB5CCNAME would be replaced with
that from the attaching user.  This conflicts with the cache used
by ktmux_helper.

Instead, use set-environment to put KRB5CCNAME in the session
environment and kinit with it before running tmux.