.golangci.yml: add nolintlint, fix found issues

The errrolint linter can finally ignore errors from Close,
and it also ignores direct comparisons of errors from x/sys/unix.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/factory_linux_test.go

@@ -86,7 +86,7 @@ func marshal(path string, v interface{}) error {
 	if err != nil {
 		return err
 	}
-	defer f.Close() //nolint: errcheck
+	defer f.Close()
 	return utils.WriteJSON(f, v)
 }