Apparently, submodules don't work correctly when both `branch` and
`shallow` are specified. It seems `git` fetches the latest commit and
_then_ tries to switch branches, which always fails:
> Unable to find refs/remotes/origin/2025.08.x revision in submodule
> path 'buildroot'
Let's just get rid of the submodule and do a shallow clone of
`buildroot` manually during the build.
