migrations: Fix S3 backend detection

internal/backend/limiter/limiter_backend.go

@@ -46,6 +46,8 @@ func (r rateLimitedBackend) Load(ctx context.Context, h restic.Handle, length in
 	})
 }
 
+func (r rateLimitedBackend) Unwrap() restic.Backend { return r.Backend }
+
 type limitedReader struct {
 	io.Reader
 	writerTo io.WriterTo

internal/backend/logger/log.go

@@ -75,3 +75,5 @@ func (be *Backend) Close() error {
 	debug.Log("  close err %v", err)
 	return err
 }
+
+func (be *Backend) Unwrap() restic.Backend { return be.Backend }

internal/backend/retry/backend_retry.go

@@ -191,3 +191,7 @@ func (be *Backend) List(ctx context.Context, t restic.FileType, fn func(restic.F
 
 	return err
 }
+
+func (be *Backend) Unwrap() restic.Backend {
+	return be.Backend
+}

internal/backend/sema/backend.go

@@ -85,3 +85,7 @@ func (be *SemaphoreBackend) Remove(ctx context.Context, h restic.Handle) error {
 
 	return be.Backend.Remove(ctx, h)
 }
+
+func (be *SemaphoreBackend) Unwrap() restic.Backend {
+	return be.Backend
+}