clientupdate: disable auto update on NixOS (#11136)

Updates #cleanup

NixOS packages are immutable and attempts to update via our tarball
mechanism will always fail as a result. Instead we now direct users to
update their nix channel or nixpkgs flake input to receive the latest
Tailscale release.

Signed-off-by: Patrick O'Doherty <patrick@tailscale.com>

clientupdate/clientupdate.go

@@ -167,6 +167,10 @@ func (up *Updater) getUpdateFunction() (fn updateFunction, canAutoUpdate bool) {
 		return up.updateWindows, true
 	case "linux":
 		switch distro.Get() {
+		case distro.NixOS:
+			// NixOS packages are immutable and managed with a system-wide
+			// configuration.
+			return up.updateNixos, false
 		case distro.Synology:
 			// Synology updates use our own pkgs.tailscale.com instead of the
 			// Synology Package Center. We should eventually get to a regular
@@ -522,6 +526,13 @@ func (up *Updater) updateArchLike() error {
 you can use "pacman --sync --refresh --sysupgrade" or "pacman -Syu" to upgrade the system, including Tailscale.`)
 }
 
+func (up *Updater) updateNixos() error {
+	// NixOS package updates are managed on a system level and not individually.
+	// Direct users to update their nix channel or nixpkgs flake input to
+	// receive the latest version.
+	return errors.New(`individual package updates are not supported on NixOS installations. Update your system channel or flake inputs to get the latest Tailscale version from nixpkgs.`)
+}
+
 const yumRepoConfigFile = "/etc/yum.repos.d/tailscale.repo"
 
 // updateFedoraLike updates tailscale on any distros in the Fedora family,