zsh: make use of corporate master.shellrc in order to access `t`
zsh: make use of corporate master.shellrc in order to access `t`

I already have `t` aliased to `tmux`, so this sets up an alias `tt` that
runs `command t`, which gives me the installed tool.

Note that all of this depends on `$LOCAL_ADMIN_SCRIPTS` being defined,
which I have done in my `~/.zshrc.local` to avoid leaking paths into
this Git repo.

wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent