[Nix-dev] Use Mac OS keychain to store ssh passphrases

[Anders Lundstedt]

(The following problem appeared when I upgraded to Mac OS Sierra,
in connection with which I did a clean install of nixpkgs.)

When I use the system SSH I do not have to enter passphrases for my
SSH keys, as they are stored in the system keychain. However if I use
the openssh provided by nixpkgs I have to enter passphrases. In
particular, git (provided by nixpkgs) always asks for passphrases
(since it uses the openssh provided by nixpkgs, I presume).

Can this be solved? Preferably such that openssh provided by nixpkgs
has access to the system keychain, but otherwise such that git uses
the system's ssh and not the one provided by nixpkgs?