This adds yubikey-agent as a package and a nixos module.
On macOS, we use `wrapProgram` to set pinentry_mac as default in PATH;
on Linux we rely on the user to set their preferred pinentry in PATH.
In particular, we use a systemd override to prefix PATH to select a
chosen pinentry program if specified.
On Linux, we need libnotify to provide the notify-send utility for
desktop notifications (such as "Waiting for Yubikey touch...").
This might work on other flavors of unix, but I haven't tested.
We reuse the programs.gnupg.agent.pinentryFlavor option for
yubikey-agent, but in doing so I hit a problem: pinentryFlavour's
default value is specified in a mkDefault, but only conditionally. We
ought to be able to pick up the pinentryFlavour whether or not gpg-agent
is running. As a result, this commit moves the default value to the
definition of programs.gnupg.agent.enable.
This solves the dependency cycle in gcr alternatively so there won't be
two gnupg store paths in a standard NixOS system which has udisks2 enabled
by default.
NixOS users are expected to use the gpg-agent user service to pull in the
appropriate pinentry flavour or install it on their systemPackages and set
it in their local gnupg agent config instead.
Co-authored-by: Florian Klink <flokli@flokli.de>
