nixos: services: lohr: fix SSH key creation

In the migration to `tmpfiles.d(5)`, I used the wrong type of file.

Using `f` would write the path to the file as its content, rather than
copy it. Unfortunately `C` and `C+` do not overwrite an existing file,
so using a symlink it the correct solution here.

This means the SSH key file must have `lohr` as an owner... Perhaps I
should make it so the service can read the file itself, rather than
rely on the filesystem location, so that I don't have to contort myself
quite so much to make it work.

modules/nixos/services/lohr/default.nix

@@ -99,7 +99,7 @@ in
         };
       };
       "${lohrHome}/.ssh/id_ed25519" = {
-        "f+" = {
+        "L+" = {
           user = "lohr";
           group = "lohr";
           mode = "0700";