clientupdate: add linux tarball updates (#9144)

As a fallback to package managers, allow updating tailscale that was
self-installed in some way. There are some tricky bits around updating
the systemd unit (should we stick to local binary paths or to the ones
in tailscaled.service?), so leaving that out for now.

Updates #6995

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

clientupdate/systemd_other.go

@@ -0,0 +1,15 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !linux
+
+package clientupdate
+
+import (
+	"context"
+	"errors"
+)
+
+func restartSystemdUnit(ctx context.Context) error {
+	return errors.ErrUnsupported
+}