build-fast: propagate/include metadata

ostree has nice flexible support for injecting metadata into commits. Cool. We've been extensively using it in various ways, e.g coreos-assembler injects basearch and rpm-ostree injects the initramfs-args.

cosa build-fast omits all that stuff. One noticeable side effect of missing the basearch is that zincati goes into a start-die-restart loop (should probably fix cosa to have "this is a fastbuild" flag or something and zincati gracefully shuts down if so?). The only real harm is the spam in the journal, but still.

And a side effect of missing the initramfs-args is that testing rpm-ostree changes in a cosa build-fast and then testing replacing the kernel in there will end up generating a broken initramfs (this took me longer than it should have to figure out, I got distracted by thinking it was selinux because for some reason we're getting a new denial in 33 when rebuilding the initramfs client side).

(I use cosa build-fast a lot)

Start by running and tracing the cosa build-fast entry point, then compare its commit metadata with the metadata produced by a regular build. Done means fast builds preserve the relevant injected metadata, including basearch and initramfs-args, without causing the reported downstream behavior.