pcr: Import guix-installer from Aur
    
    Building Guix from within Parabola is better as:
    - It rebuilds guix from source, so we don't rely on more binaries.
      In addition it enables to test reproducible builds when built from
      Parabola.
    - We end up with less space being taken by dependencies at the beginning
      as the dependencies are shared with other Parabola packages.
    
    However building Guix didn't work well on i686 for instance, and it's also
    a good idea to provide guix's installer as well as there are different
    tradeoffs being made between the installer packages built within Parabola.
    
    For instance Guix binaries are built against Guile3 which isn't in Parabola
    yet.
    
    Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>