homebrew: Load 'HOMEBREW_' prefixed variables only

Load 'HOMEBREW_' prefixed variables only. Avoid loading 'PATH'
related variables as they are already handled in standard zsh 
configuration.

modules/homebrew/init.zsh

@@ -14,10 +14,11 @@ fi
 # Variables
 #
 
-# Load standard Homebrew shellenv into the shell session.
+# Load standard Homebrew shellenv into the shell session. 
-# `brew shellenv` is relatively new, guard for legacy Homebrew.
+# Load 'HOMEBREW_' prefixed variables only. Avoid loading 'PATH' related
+# variables as they are already handled in standard zsh configuration.
 if (( $+commands[brew] )); then
-  eval "$(brew shellenv 2> /dev/null)"
+  eval "${(@M)${(f)"$(brew shellenv 2> /dev/null)"}:#export HOMEBREW*}"
 fi
 
 #