module-build-macros: Use /etc instead of %_sysconfdir
    
    Commit 42aef7d5d2 moved the macros installed by module-build-macros
    from %_rpmconfdir (which is unaffected by %_prefix) to %_sysconfdir/rpm
    (which follows %_prefix). This interacts badly with the Flatpak module
    macros which change %_prefix to /app - when flatpak-rpm-macros is
    in the buildroot while module-build-macros is built, module-build-macros
    installs its macros to /app and they aren't found when building the
    rest of the packages in the module.
    
    Hardcoding /etc rather than using %_sysconfdir fixes this.