Impermanence does not work when `system.etc.overlay.mutable = false`

[Guanran928]

configuration:

  system.etc.overlay.enable = true;
  system.etc.overlay.mutable = false;
  services.userborn.enable = true;

systemd:

$ systemctl --failed
  UNIT                                                              LOAD   ACTIVE SUB    DESCRIPTION                                                                                          
● persist-\x27-persist-etc-ssh-ssh_host_ed25519_key.pub\x27.service loaded failed failed Bind mount or link '/persist/etc/ssh/ssh_host_ed25519_key.pub' to '/etc/ssh/ssh_host_ed25519_key.pub'
● persist-\x27-persist-etc-ssh-ssh_host_ed25519_key\x27.service     loaded failed failed Bind mount or link '/persist/etc/ssh/ssh_host_ed25519_key' to '/etc/ssh/ssh_host_ed25519_key'
● persist-\x27-persist-etc-ssh-ssh_host_rsa_key.pub\x27.service     loaded failed failed Bind mount or link '/persist/etc/ssh/ssh_host_rsa_key.pub' to '/etc/ssh/ssh_host_rsa_key.pub'
● persist-\x27-persist-etc-ssh-ssh_host_rsa_key\x27.service         loaded failed failed Bind mount or link '/persist/etc/ssh/ssh_host_rsa_key' to '/etc/ssh/ssh_host_rsa_key'

Legend: LOAD   → Reflects whether the unit definition was properly loaded.
        ACTIVE → The high-level unit activation state, i.e. generalization of SUB.
        SUB    → The low-level unit activation state, values depend on unit type.

4 loaded units listed.

log:

Aug 31 01:48:08 dust zhwr6q6s9q05c12015397d2jsw8zfd8r-impermanence-mount-file[1113]: touch: cannot touch '/etc/ssh/ssh_host_rsa_key': Read-only file system
Aug 31 01:48:08 dust zhwr6q6s9q05c12015397d2jsw8zfd8r-impermanence-mount-file[1096]: Error when executing touch "$mountPoint" at line 39!
Aug 31 01:48:08 dust systemd[1]: persist-\x27-persist-etc-ssh-ssh_host_rsa_key\x27.service: Main process exited, code=exited, status=1/FAILURE

A workaround:

environment.etc."secureboot".source = pkgs.emptyDirectory;
environment.etc."ssh/ssh_host_ed25519_key.pub".source = pkgs.emptyFile;