Now that we have the kernel build infrastructure at https://github.com/edera-dev/kernels
it makes sense to drop building the kernel and download the kernel images directly.
This change introduces a ./hack/kernel/fetch.sh script which is backed by crates/build
We utilize the OCI infrastructure itself to download the kernel image. The DEV guide
has been updated to include calling the fetch script, and the OS builder now uses this
method instead. Due to the lack of need for the kernel build infra to exist here now,
it has also been removed. This should significantly speed up full builds.
This change will also enable us to turn on os build workflows for all PRs. We should
likely make the OS status checks required once this is merged.
