[vim] Export $FZF_HEIGHT for previewer scripts

Preview script cannot properly determine the height of fzf finder if
`--height` option is used.

https://github.com/junegunn/fzf.vim/issues/361

plugin/fzf.vim

@@ -379,7 +379,9 @@ try
     let use_term = 0
   endif
   if use_height
-    let optstr .= ' --height='.s:calc_size(&lines, dict.down, dict)
+    let height = s:calc_size(&lines, dict.down, dict)
+    let $FZF_HEIGHT = height
+    let optstr .= ' --height='.height
   elseif use_term
     let optstr .= ' --no-height'
   endif