Describe the bug

When running nix-rebuild, it prints a bunch of ignoring generation 'None'.

No new boot entries are being created.

Steps To Reproduce

run sudo nixos-rebuild switch --upgrade

ignoring generation 'None' in the list of boot entries because of the following error:
65
ignoring generation 'None' in the list of boot entries because of the following error:
66
ignoring generation 'None' in the list of boot entries because of the following error:
67

and so on...

Expected behavior

it should create new boot entries

nix-env --version output

2.6.0

Additional context

in /boot/loader/entries/, there is a bunch of empty files like nixos-generation-65.conf.tmp.

I figure out that /boot/ is full by trying to write the entries manually. nixos-upgrade should figure that out and give feedback with advice on cleaning up kernels.