add ${ENV_ROOT}/shims

When tmux is started, the execution result of the
'$(${ENV_ROOT}/bin/${env} init -)' is 'source
$HOME/.anyenv/envs/plenv/libexec/../completions/plenv.zsh' instead of
export PATH additional shims.
So, is not available *env.

As a result of this commit, it will be read twice the shims, but I do
not care.
e.g.)
$HOME/.anyenv/envs/rbenv/bin
$HOME/.anyenv/envs/rbenv/shims
$HOME/.anyenv/envs/plenv/bin
$HOME/.anyenv/envs/plenv/shims
$HOME/.anyenv/envs/rbenv/shims
$HOME/.anyenv/envs/plenv/shims