If you want to be able to use OpenSC with ssh-agent, you need to be able
to add it to the ssh-agent whitelist. This adds an option,
agentPKCS11Whitelist, that exposes the option.
Note that I currently work around this by injecting the parameter into
the agentTimeout option:
programs.ssh.agentTimeout = "1h -P ${pkgs.opensc}/lib/opensc-pkcs11.so";
but I feel that a proper option would be better :)
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