self-update: Fix handling of --output on windows

The code always assumed that the upgrade happens in place. Thus writing
the upgrade to a separate file fails, when trying to remove the file
stored at that location.

internal/selfupdate/download_windows.go

@@ -7,11 +7,18 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+
+	"github.com/restic/restic/internal/errors"
 )
 
 // Rename (rather than remove) the running version. The running binary will be locked
 // on Windows and cannot be removed while still executing.
 func removeResticBinary(dir, target string) error {
+	// nothing to do if the target does not exist
+	if _, err := os.Stat(target); errors.Is(err, os.ErrNotExist) {
+		return nil
+	}
+
 	backup := filepath.Join(dir, filepath.Base(target)+".bak")
 	if _, err := os.Stat(backup); err == nil {
 		_ = os.Remove(backup)