With the rpm-4.14.3 on SLE-15-SP3, using rpm.open through pcall can
not access the db and SecureBoot efi variable files. We got the
following message when the pretrans lua script is running:
WARNING: Attempt to open db EFI variable file failed. Error message:
attempt to call a nil value
Using io.open instead of pcall(rpm.open can workaround this issue.