rpm: add RPM_FORCE_DEBIAN= env var as alternative to --force-debian

Command line parameters cannot be passed to dnf, so add support for an env var to bypass the warning on package install.

In mkosi we use the local dnf package to bootstrap new chroots, which uses the local rpm package, and shows this warning on every package being installed, despite the fact that it is perfectly safe to do so as it is not installing on the host, but in the chroot.

Closes: #1051223