nixos/filesystems: don't chown /run/keys recursively

3c74e48d was a bit too much, it updated
permissions of all files recursively, causing files to be readable by
the group.

This isn't a problem immediately after bootup, but on a new activation,
as tmpfiles.d get restarted then, updating the permission bits of
now-existing files.

This updates the `Z` to be a `z` (the non-recursive variant), and adds a
`d` to ensure a directory is created (which should be covered by the
initrd shell script anyway)