homebrew: Simplify caching brew shellenv

Avoid checking age of cache file and just test for its existence.
While this is simpler, this also avoids depending on having
`EXTENDED_GLOB` enabled.

modules/homebrew/init.zsh

@@ -21,7 +21,17 @@ fi
 # Load 'HOMEBREW_' prefixed variables only. Avoid loading 'PATH' related
 # variables as they are already handled in standard zsh configuration.
 if (( $+commands[brew] )); then
-  eval "${(@M)${(f)"$(brew shellenv 2> /dev/null)"}:#export HOMEBREW*}"
+  cache_file="${XDG_CACHE_HOME:-$HOME/.cache}/prezto/brew-shellenv-cache.zsh"
+  if [[ "$commands[brew]" -nt "$cache_file" \
+      || "${ZDOTDIR:-$HOME}/.zpreztorc" -nt "$cache_file" \
+      || ! -s "$cache_file" ]]; then
+    mkdir -p "$cache_file:h"
+    # Cache the result.
+    echo "${(@M)${(f)"$(brew shellenv 2> /dev/null)"}:#export HOMEBREW*}" >! "$cache_file" 2> /dev/null
+  fi
+
+  source "$cache_file"
+  unset cache_file
 fi
 
 #