Fix autojump script to support Nix installations

plugins/autojump/autojump.plugin.zsh

 if [ $commands[autojump] ]; then # check if autojump is installed
   if [ -f $HOME/.autojump/etc/profile.d/autojump.zsh ]; then # manual user-local installation
     . $HOME/.autojump/etc/profile.d/autojump.zsh
+  elif [ -f $HOME/.nix-profile/etc/profile.d/autojump.zsh ]; then # nix installation
+    . $HOME/.nix-profile/etc/profile.d/autojump.zsh
   elif [ -f /usr/share/autojump/autojump.zsh ]; then # debian and ubuntu package
     . /usr/share/autojump/autojump.zsh
   elif [ -f /etc/profile.d/autojump.zsh ]; then # manual installation