fix(terraform): use faster method to get workspace (#9709)

Also add tf_prompt_info to the list of prompt functions so theme writers are aware of it

lib/prompt_info_functions.zsh

@@ -18,6 +18,7 @@ function chruby_prompt_info \
   vi_mode_prompt_info \
   virtualenv_prompt_info \
   jenv_prompt_info \
+  tf_prompt_info \
 {
   return 1
 }