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
 }

plugins/terraform/terraform.plugin.zsh

@@ -2,8 +2,8 @@ function tf_prompt_info() {
     # dont show 'default' workspace in home dir
     [[ "$PWD" == ~ ]] && return
     # check if in terraform dir
-    if [ -d .terraform ]; then
-      workspace=$(terraform workspace show 2> /dev/null) || return
+    if [[ -d .terraform && -r .terraform/environment  ]]; then
+      workspace=$(cat .terraform/environment) || return
       echo "[${workspace}]"
     fi
 }