cmd/tailscale/cli: fix ssh CLI command breaking the Wasm build

Adds a stub for syscall.Exec when GOOS=js. We also had a separate branch
for Windows, might as well use the same mechanism there too.

For #3157

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

cmd/tailscale/cli/ssh_exec.go

@@ -0,0 +1,21 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !js && !windows
+// +build !js,!windows
+
+package cli
+
+import (
+	"errors"
+	"os"
+	"syscall"
+)
+
+func execSSH(ssh string, argv []string) error {
+	if err := syscall.Exec(ssh, argv, os.Environ()); err != nil {
+		return err
+	}
+	return errors.New("unreachable")
+}