Use better randomseed for Lua too

see e985b7a0bf5ddf4999330a5a1a878654f68b254e

This solves a concrete issue with async workers generating the same
random numbers (as discovered in #14518).

builtin/init.lua

@@ -26,7 +26,15 @@ do
 		core.print = nil -- don't pollute our namespace
 	end
 end
-math.randomseed(os.time())
+
+do
+	-- Note that PUC Lua just calls srand() which is already initialized by C++,
+	-- but we don't want to rely on this implementation detail.
+	local seed = 1048576 * (os.time() % 1048576)
+	seed = seed + core.get_us_time() % 1048576
+	math.randomseed(seed)
+end
+
 minetest = core
 
 -- Load other files