So the virtualbox-guest-dkms package was installed but no module was
built. I think that's explained by this comment:
"# Any -dkms package must be installed *after* dkms to be properly registered"
… which is the case usually (my build logs say that dkms is installed
by the 50-dkms hook), but apparently your new hook makes this
requirement not satisfied anymore. I suggest you add your code to
some relevant place in 50-dkms instead of creating a new hook that
runs earlier.