sshd: Don't remove symlinks to host key files

If a host key file is a symlink pointing to an as of yet non-existent
file, we don't want to remove it, but instead follow the symlink and
create the file at that location.

See https://github.com/nix-community/impermanence/issues/101 for more
information on the issue the original behavior creates.

nixos/modules/services/networking/ssh/sshd.nix

@@ -442,7 +442,9 @@ in
 
                 ${flip concatMapStrings cfg.hostKeys (k: ''
                   if ! [ -s "${k.path}" ]; then
-                      rm -f "${k.path}"
+                      if ! [ -h "${k.path}" ]; then
+                          rm -f "${k.path}"
+                      fi
                       ssh-keygen \
                         -t "${k.type}" \
                         ${if k ? bits then "-b ${toString k.bits}" else ""} \