var shell = WScript.CreateObject("WScript.Shell");
var env = shell.Environment("PROCESS");

function sub_fn(all, name) {
    return env(name);
}

var pat = /{(.*?)}/g;
var from = WScript.StdIn.ReadAll();
WScript.StdOut.Write(from.replace(pat, sub_fn));

// Or:
// WScript.StdOut.Write(shell.ExpandEnvironmentStrings(from))