Use GitHub release tarball instead of zlib.net

Because zlib.net only provides the latest version of zlib and the old
versions are not available after the new version is released.

Author: kateinoigakukun

lib/ruby_wasm/build/product/zlib.rb

@@ -46,7 +46,7 @@ def build(executor)
                       "-o",
                       tarball_path,
                       "-L",
-                      "https://zlib.net/zlib-#{ZLIB_VERSION}.tar.gz"
+                      "https://github.com/madler/zlib/releases/download/v#{ZLIB_VERSION}/zlib-#{ZLIB_VERSION}.tar.gz"
       executor.system "tar",
                       "xzf",
                       tarball_path,