Add missing lib to Nix flake

[Hasnep]

This was accidentally removed in a recent change and broke the flake.

[StevenBlack]

Thank you very much for this Hannes @Hasnep.

I'm just going to double check with those involved.

Bobbe @30350n and @toastal any thoughts on this?

[30350n]

LGTM! This should be merged, without out it the current flake is completely broken. (Seems like the line in question came from a PR merged after #2813 s creation, which is why this slipped through. We should have checked #2813 for correctness before merging again.)

[StevenBlack]

Thank you Bobbe @30350n and Hannes @Hasnep.

Merging!

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