zsh: Unset `warn_create_global`
zsh: Unset `warn_create_global`

`warn_create_global` causes a warning message to be printed when a
global parameter is created by assignment in a function, which should
usually have been declared with `local`. Unfortunately this occurs often
in the LHCb setup scripts, causing a flood of warning messages when they
are sourced.

srstevenson/dotfiles,srstevenson/dotfiles,srstevenson/dotfiles