Copy __ENV so that 2 VUs don't share the same one

as in #799(which was about the same issue with setup data), we
don't actually have problems with a VU seeing changes from
previous iterations. It will be too expensive to constantly copy, it
won't panic and there are no issues with distributed execution.

js/bundle.go

@@ -255,7 +255,11 @@ func (b *Bundle) instantiate(rt *goja.Runtime, init *InitContext) error {
 	_ = module.Set("exports", exports)
 	rt.Set("module", module)
 
-	rt.Set("__ENV", b.Env)
+	env := make(map[string]string, len(b.Env))
+	for key, value := range b.Env {
+		env[key] = value
+	}
+	rt.Set("__ENV", env)
 	rt.Set("console", common.Bind(rt, newConsole(), init.ctxPtr))
 
 	*init.ctxPtr = common.WithRuntime(context.Background(), rt)

js/bundle_test.go

@@ -740,6 +740,48 @@ func TestBundleEnv(t *testing.T) {
 	}
 }
 
+func TestBundleNotSharable(t *testing.T) {
+	data := `
+		export default function() {
+			if (__ITER == 0) {
+				if (typeof __ENV.something !== "undefined") {
+					throw new Error("invalid something: " + __ENV.something + " should be undefined");
+				}
+				__ENV.something = __VU;
+			} else if (__ENV.something != __VU) {
+				throw new Error("invalid something: " + __ENV.something+ " should be "+ __VU);
+			}
+		}
+	`
+	b1, err := getSimpleBundle("/script.js", data)
+	if !assert.NoError(t, err) {
+		return
+	}
+
+	b2, err := NewBundleFromArchive(b1.makeArchive(), lib.RuntimeOptions{})
+	if !assert.NoError(t, err) {
+		return
+	}
+
+	bundles := map[string]*Bundle{"Source": b1, "Archive": b2}
+	vus, iters := 10, 1000
+	for name, b := range bundles {
+		b := b
+		t.Run(name, func(t *testing.T) {
+			for i := 0; i < vus; i++ {
+				bi, err := b.Instantiate()
+				bi.Runtime.Set("__VU", i)
+				require.NoError(t, err)
+				for j := 0; j < iters; j++ {
+					bi.Runtime.Set("__ITER", j)
+					_, err := bi.Default(goja.Undefined())
+					assert.NoError(t, err)
+				}
+			}
+		})
+	}
+}
+
 func TestBundleMakeArchive(t *testing.T) {
 	testCases := []struct {
 		cm      compiler.CompatibilityMode