install to-filesystem: look into the container for grub configs with `--src-imgref`

[jbtrystram]

When using --src-imgref, bootupd fetches the static configs from the host rather than the destination container.

An easy way around would be to chroot to the deployed path before the bootupd invocation (this would have the added bonus of using the bootupd binary that is shipped into the image rather than the one in the host).

e.g like in https://github.com/osbuild/osbuild/blob/main/stages/org.osbuild.bootupd

When building FCOS i got only the configs that exist in COSA rather than what is in the container (thus missing the ignition fragment)

Related: #1455

[jbtrystram]

Hmmm maybe it's more a bug in bootupd because we already specify --src-root.

[cgwalters]

Yeah this is probably a bootupd bug.

That said it would also be really nice always run bootupd from the target root, we can totally do that, it just requires some plumbing work.

[cgwalters]

This was fixed by coreos/bootupd#990