builder: Check that command exists after build
builder: Check that command exists after build

This keeps us from creating a non-working flatpak.

matthiasclasen/flatpak,GeorgesStavracas/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,GeorgesStavracas/flatpak,amigadave/flatpak,GeorgesStavracas/flatpak,amigadave/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,amigadave/flatpak,flatpak/flatpak,handsome-feng/flatpak,amigadave/flatpak,matthiasclasen/flatpak,GeorgesStavracas/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,flatpak/flatpak,flatpak/flatpak,handsome-feng/flatpak