cmd/tailscale: stop defaulting ssh username to local username (#19358)

Prevent tailscale ssh from automatically adding a username when
connecting to a server, only forward one if provided. The previous
behaviour prevented username overrides in the ssh configuration, since
the provided username takes precedence to the configured one.

This also keeps the tailscale ssh a thin wrapper around ssh by not
adding any extra arguments unless required.

Fixes #19357

Signed-off-by: Örjan Fors <o@42mm.org>

cmd/tailscale/cli/ssh.go

@@ -11,7 +11,6 @@
 	"log"
 	"net/netip"
 	"os"
-	"os/user"
 	"path/filepath"
 	"runtime"
 	"slices"
@@ -59,11 +58,7 @@ func runSSH(ctx context.Context, args []string) error {
 	username, host, ok := strings.Cut(arg, "@")
 	if !ok {
 		host = arg
-		lu, err := user.Current()
-		if err != nil {
-			return nil
-		}
-		username = lu.Username
+		username = ""
 	}
 
 	st, err := localClient.Status(ctx)
@@ -146,7 +141,11 @@ func runSSH(ctx context.Context, args []string) error {
 	// to use a different one, we'll later be making stock ssh
 	// work well by default too. (doing things like automatically
 	// setting known_hosts, etc)
-	argv = append(argv, username+"@"+hostForSSH)
+	if username == "" {
+		argv = append(argv, hostForSSH)
+	} else {
+		argv = append(argv, username+"@"+hostForSSH)
+	}
 
 	argv = append(argv, argRest...)