We should have --rpm-cmd option

[praiskup]

Something that will provide similar functionality like --dnf-cmd.

In #1476 I realized that, on F40, we can not easily do:
$ mock --no-bootstrap-chroot -r centos-stream-9-x86_64 --shell 'rpm -qa'

The thing is that --shell provides no abstraction for the rpm call. Mock uses dnf4 on host to install to buildroot, it implies that the rpmdb uses sqlite - but the inner RPM in c9s is not able to read it.