lightdm_gtk_greeter: fix configureFlags to not be one long string

For whatever reason, setting `configureFlagsArray` causes
the configure invocation to escape spaces between parameters:

configure flags: --disable-static --disable-dependency-tracking --prefix=/nix/store/4bn8dsr743dp0g5j020jf8why5hkld3p-lightdm-gtk-greeter-2.0.6 --localstatedir=/var\ --sysconfdir=/etc\ --disable-indicator-services-command\ --enable-at-spi-command=/nix/store/li9bsxl8qja8qw0pa70xd572gw5hqsdm-at-spi2-core-2.30.1/libexec/at-spi-bus-launcher\ \ --launch-immediately

This actually doesn't break the build (!) but it does break
lightdm pretty badly for me and it's not hard to imagine why :).

Instead, use configureFlags as normal but just append the command
in `preConfigure` directly, so we can preserve the space.