Add an option to drop /usr/share/rpm rpmdb legacy

I'd swear there was an issue for this but I can't find it. This relates to https://github.com/coreos/rpm-ostree/issues/5250 among others.

Right now we hardcode having the rpmdb in `/usr/share/rpm` for compatibility with prior state. But we should add a build option usable for current Fedora and RHEL10 that relies on the default `/usr/lib/sysimage/rpm`.