nixos-ssh: set SSH_ASKPASS globally and not just on interactive shells

If we limit SSH_ASKPASS to interactive shells, users are unable to trigger
the ssh-passphrase dialog from their desktop environment autostart scripts.
Usecase: I call ssh-add during my desktop environment autostart and want to have
the passphrase dialog immediately after startup.
For this to work, SSH_ASKPASS needs to be propagated properly on
non-interactive shells.