allowing z plugin to use a custom path

plugins/z/z.fish

 function z
-  cd (bash -c 'source /usr/local/etc/profile.d/z.sh; _z $0; echo $PWD' $argv)
+  cd (bash -c 'source $Z_SCRIPT_PATH; _z $0; echo $PWD' $argv)
 end

plugins/z/z.load

+if test -z "$Z_SCRIPT_PATH"
+  set -x Z_SCRIPT_PATH /usr/local/etc/profile.d/z.sh
+end
+
 function __check_z --on-variable PWD --description 'Setup z on directory change'
   status --is-command-substitution; and return
 
-  bash -c "source /usr/local/etc/profile.d/z.sh; _z --add `pwd -P`"
+  bash -c "source $Z_SCRIPT_PATH; _z --add `pwd -P`"
 end