[build2] Set RPM epoch

[Boris Kolpackov]

Paul Stath via users <users at build2.org> writes:

> I had expected that since the distribution specific (fedora-version)
> attribute was being set that this logic would not apply and the <epoch>
> "So we shift this value range" logic would not apply.

Yes, you are right and I was confused: fedora-version is the Fedora-native
version (as opposed to bpkg version) so no range shift applies and the 1:
epoch should work (and appears to work per Karen's testing).