Switch to the built-in rpm-ostree rechunker

[tulilirockz]

I dont really see anything wrong with the current rechunker but id much rather rely on upstream doing it than hhd-dev. Got this implemented on #275 and honestly it just seems like a no-brainer to me. It currently has some downsides tho, that being that it is a bit less intelligent (because it doesnt group commonly used packages) and it doesnt have a few selinux workarounds that hhd-dev/rechunk has.

[p5]

We're currently using hhd-dev/rechunk to split our images into smaller layers, making updates easier to download.

rpm-ostree has recently added built-in support for rechunking.

A general rule-of-thumb for bluefin-lts has been to miminise the number of external dependencies. These include ensuring all packages added are either in EPEL or ublue-os/staging. Personally, I don't see GitHub Actions being very different - using the vendor's own actions should be preferred.

As @tulilirockz mentioned, there are a couple downsides, but this provides a more supported method of rechunking, so it seems like a good switch to me

[tulilirockz]

The test image we did with that PR seems to not boot right now because of (maybe) some SELinux issue with Dbus.

[tulilirockz]

Alright, managed to debug a few things. This is for sure not SELinux causing it. Might be something to do with how the image is extracted into a rootfs?