s3: ensure we can set upload cutoff that we use for Rclone provider

This is a workaround to make the new multipart upload integration
tests pass.

backend/s3/s3.go

@@ -3004,7 +3004,9 @@ func checkUploadCutoff(cs fs.SizeSuffix) error {
 }
 
 func (f *Fs) setUploadCutoff(cs fs.SizeSuffix) (old fs.SizeSuffix, err error) {
-	err = checkUploadCutoff(cs)
+	if f.opt.Provider != "Rclone" {
+		err = checkUploadCutoff(cs)
+	}
 	if err == nil {
 		old, f.opt.UploadCutoff = f.opt.UploadCutoff, cs
 	}