Trees: Add 'snowy' group for pine sapling snow detection

Use the 'snowy' group in the 'is snow nearby' function instead of
default nodes. Often snow nodes are not from the default mod.

mods/default/trees.lua

@@ -27,8 +27,7 @@ end
 -- 'is snow nearby' function
 
 local function is_snow_nearby(pos)
-	return minetest.find_node_near(pos, 1,
-		{"default:snow", "default:snowblock", "default:dirt_with_snow"})
+	return minetest.find_node_near(pos, 1, {"group:snowy"})
 end