Automate using latest release of gentoo_prefix_ci

[awesomebytes]

Here: https://github.com/awesomebytes/ros_overlay_on_gentoo_prefix/blob/master/ros_base/Dockerfile#L40

[awesomebytes]

@Xia0ben I felt like procrastinating a bit and already made a one liner to be able to download the latest release of Gentoo Prefix to use as base image for this repo (and the 32b one).

You can find it in this commit: bfd34f1
Just some curl + grep + cut magic :)

[awesomebytes]

@Xia0ben I've updated the repo (and the 32b repo too https://github.com/awesomebytes/ros_overlay_on_gentoo_prefix_32b/) with pulling the latest release, and also applying a few patches to actually build ros_base and maybe desktop too.