nixos/gdm: Fix fingerprint auth rules

We introduced the gdm-fingerprint.pam in 9d41fe6fcc4df838a56b1cfb2512b65e5e655958.

We used the [upstream Arch config] as a template, which contains an extended control field that jumps over **one** immediately-following `auth` rule unless `pam_gdm.so` succeeds.

But we decided to not include `pam_gnome_keyring.so` so there was no rule to skip over, resulting in a broken control flow and the PAM module failing with “PAM bad jump in stack”, breaking the fingerprint authentication in GDM.

Let’s actually add `pam_gnome_keyring.so`, like the Arch config does. Because we are creating the PAM file using the `text` option, `security.pam.services.gdm-fingerprint.enableGnomeKeyring` does not do anything so we need to do it manually.

For the case where gnome-keyring is not enabled, we could add a no-op rule like `optional pam_permit.so` after `pam_gdm.so` so that the branching always has something to jump over but it will be simpler to just make the both conditional. There are no further `auth` rules that could benefit from `pam_gdm.so` doing something so it should be fine.

Unlike in Arch, we are not going to invoke `pam_gnome_keyring.so` in a `session` rule since that is already done by the included `login` module.

[upstream Arch config]: https://gitlab.gnome.org/GNOME/gdm/-/blob/81ee658c11381912131dd4a29e84190f7f9cd039/data/pam-arch/gdm-fingerprint.pam

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