Move package note generation to build preamble
    
    We would try to regenerate the file whenever set_build_flags was used.
    For packages which call %configure multiple times this would mean a
    few times during build, and also in the build and check
    preambles. Let's just simplify this and call it only in the build preamble.
    
    https://src.fedoraproject.org/rpms/package-notes/pull-request/1
    changes %_generate_package_note_file to be unconditional. This means we'll
    always rewrite the file and will not use a stale version. (But the two
    fixes are independent, even though they make the most sense together.)