cmd/tailscale: allow use of flags in gokrazy

Enable use of command line arguments with tailscale cli on gokrazy. Before
this change using arguments like "up" would cause tailscale cli to be
repeatedly restarted by gokrazy process supervisor.

We never want to have gokrazy restart tailscale cli, even if user would
manually start the process.

Expected usage is that user creates files:

flags/tailscale.com/cmd/tailscale/flags.txt:

    up

flags/tailscale.com/cmd/tailscaled/flags.txt:

    --statedir=/perm/tailscaled/
    --tun=userspace-networking

Then tailscale prints URL for user to log in with browser.

Alternatively it should be possible to use up with auth key to allow
unattended gokrazy installs.

Signed-off-by: Joonas Kuorilehto <joneskoo@derbian.fi>
(cherry picked from commit c1b3500a05)

cmd/tailscale/cli/cli.go

@@ -125,16 +125,21 @@ func CleanUpArgs(args []string) []string {
 }
 
 // Run runs the CLI. The args do not include the binary name.
-func Run(args []string) error {
+func Run(args []string) (err error) {
 	if len(args) == 1 && (args[0] == "-V" || args[0] == "--version") {
 		args = []string{"version"}
 	}
-	if runtime.GOOS == "linux" && distro.Get() == distro.Gokrazy && len(args) == 0 &&
+	if runtime.GOOS == "linux" && distro.Get() == distro.Gokrazy &&
 		os.Getenv("GOKRAZY_FIRST_START") == "1" {
+		defer func() {
 			// Exit with 125 otherwise the CLI binary is restarted
 			// forever in a loop by the Gokrazy process supervisor.
 			// See https://gokrazy.org/userguide/process-interface/
+			if err != nil {
+				log.Println(err)
+			}
 			os.Exit(125)
+		}()
 	}
 
 	var warnOnce sync.Once
@@ -201,7 +206,7 @@ change in the future.
 		}
 	})
 
-	err := rootCmd.Run(context.Background())
+	err = rootCmd.Run(context.Background())
 	if tailscale.IsAccessDeniedError(err) && os.Getuid() != 0 && runtime.GOOS != "windows" {
 		return fmt.Errorf("%v\n\nUse 'sudo tailscale %s' or 'tailscale up --operator=$USER' to not require root.", err, strings.Join(args, " "))
 	}