package/zsh: security bump to version 5.8.1

Fixes the following security issue:

- CVE-2021-45444: In zsh before 5.8.1, an attacker can achieve code
  execution if they control a command output inside the prompt, as
  demonstrated by a %F argument.  This occurs because of recursive
  PROMPT_SUBST expansion.

The 5.8.1 release is not listed in MD5SUM, so drop the md5 hash.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>