clientupdate: implement update for Unraid (#10344)

Use the [`plugin`
CLI](https://forums.unraid.net/topic/72240-solved-is-there-a-way-to-installuninstall-plugins-from-script/#comment-676870)
to fetch and apply the update.

Updates https://github.com/tailscale/tailscale/issues/10184

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

ipn/ipnlocal/c2n.go

@@ -395,13 +395,18 @@ func findCmdTailscale() (string, error) {
 		if self == "/usr/local/sbin/tailscaled" || self == "/usr/local/bin/tailscaled" {
 			ts = "/usr/local/bin/tailscale"
 		}
-		if distro.Get() == distro.QNAP {
+		switch distro.Get() {
+		case distro.QNAP:
 			// The volume under /share/ where qpkg are installed is not
 			// predictable. But the rest of the path is.
 			ok, err := filepath.Match("/share/*/.qpkg/Tailscale/tailscaled", self)
 			if err == nil && ok {
 				ts = filepath.Join(filepath.Dir(self), "tailscale")
 			}
+		case distro.Unraid:
+			if self == "/usr/local/emhttp/plugins/tailscale/bin/tailscaled" {
+				ts = "/usr/local/emhttp/plugins/tailscale/bin/tailscale"
+			}
 		}
 	case "windows":
 		ts = filepath.Join(filepath.Dir(self), "tailscale.exe")