fix(shell-init): suppress cd output

[ajbw]

If cd causes output on stdout (e.g., via the use of noisy chpwd hooks on zsh), then the resultant PATH exported to the shell will be polluted with that output, and safe-chain will not function. To prevent this, this PR updates init-posix.sh and unix-wrapper.template.sh to suppress any output on stdout during cd.

You can reproduce the original issue by configuring .zshrc as follows and launching a new shell:

PATH="/bin:/usr/bin:/usr/local/bin" # minimal PATH
myfunc() {
  echo "blah"
}
autoload -U add-zsh-hook
add-zsh-hook chpwd myfunc
set -x
source ~/.safe-chain/scripts/init-posix.sh # Safe-chain bash initialization script
set +x