Move GPG agent availability check to the top

plugins/gpg-agent/init.zsh

@@ -6,12 +6,12 @@
 #   Sorin Ionescu <sorin.ionescu@gmail.com>
 #
 
-_gpg_env="$HOME/.gnupg/gpg-agent.env"
-
 if (( ! $+commands[gpg-agent] )); then
   return
 fi
 
+_gpg_env="$HOME/.gnupg/gpg-agent.env"
+
 function _gpg-agent-start() {
   gpg-agent --daemon --enable-ssh-support --write-env-file "${_gpg_env}" > /dev/null
   chmod 600 "${_gpg_env}"