nixos/kanidm: allow unixd-tasks to read /etc/{passwd/group/shadow}

This seems needed to allow map_group to work

fixes
process_etc_passwd_group [ 107µs | 100.00% ]
┕━ 🚨 [error]: | err: Os { code: 2, kind: NotFound, message: "No such file or directory" }

nixos/modules/services/security/kanidm.nix

···
           "-/etc/resolv.conf"
           "-/etc/nsswitch.conf"
           "-/etc/hosts"
+          "-/etc/passwd"
+          "-/etc/group"
+          "-/etc/shadow"
           "-/etc/localtime"
           "-/etc/kanidm"
           "-/etc/static/kanidm"