builder: Fix unused return value warnings
builder: Fix unused return value warnings

This just adds a (void) cast to avoid the warnings.

amigadave/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,amigadave/flatpak,GeorgesStavracas/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,alexlarsson/xdg-app,handsome-feng/flatpak,alexlarsson/xdg-app,flatpak/flatpak,amigadave/flatpak,GeorgesStavracas/flatpak,alexlarsson/xdg-app,handsome-feng/flatpak,flatpak/flatpak,handsome-feng/flatpak,flatpak/flatpak,alexlarsson/xdg-app,GeorgesStavracas/flatpak,matthiasclasen/flatpak,GeorgesStavracas/flatpak,amigadave/flatpak,handsome-feng/flatpak,flatpak/flatpak,matthiasclasen/flatpak