fedora-live-base.ks: do not hardcode fedora-release in %post
    
    Do not hardcode the fedora-release package name in the %post scriptlet
    (in the place that imports the Fedora RPM GPG key). Instead, use
    "--whatprovides system-release", which is strictly equivalent if
    fedora-release is used, but also works when the *-release package is
    named differently.
    
    This makes live easier for remixes, which have to replace fedora-release
    with generic-release or ${remixname}-release. They would otherwise have
    to edit fedora-live-base.ks.