two step evaluation of prompt to reduce time on setting universal var…

…iable

Unfortunately this doesn't fix the error:
`<W> fish: Locking the universal var file took too long (1.004 seconds).`
I think that error is caused by too many threads waiting on the
universal file lock:
https://github.com/fish-shell/fish-shell/blob/master/src/env_universal_common.cpp#L569

conf.d/__async_prompt.fish

@@ -65,7 +65,7 @@ and begin
         set st $status
 
         for func in (__async_prompt_config_functions)
-            __async_prompt_config_inherit_variables | __async_prompt_spawn $st 'set -U __async_prompt_'$func'_text_'(__async_prompt_pid)' ('$func')'
+            __async_prompt_config_inherit_variables | __async_prompt_spawn $st 'set -l prompt ('$func'); and set -U __async_prompt_'$func'_text_'(__async_prompt_pid)' $prompt'
             function '__async_prompt_'$func'_handler' --on-process-exit (jobs -lp | tail -n1)
                 kill -WINCH (__async_prompt_pid)
                 sleep 0.1