I finally migrated my home configuration to guix home.  However, it seems
guix home creates all symlinks with 777 permissions.  This causes problems
with openssh as it will not recognize my ~/.ssh/authorized_keys.  It seems
the directories have reasonable permissions (maybe because they already
existed?), but it seems like someone could in theory edit the symlinks
in-place (though I wasn't able to figure that out).

I formulated based on the example in Section 11.1 of the devel user
manual.  You should be able to recreate the problem with (replacing <your
ssh public key here>):

(home-environment
  (services
    (list
      (simple-service
       'my-home-files-service
       home-files-service-type
       (list
         `("ssh/authorized_keys"
            ,(plain-file
               "home-authorized-keys"
               "<your ssh public key here>")))))))