Fix the nix flake

[Nadrieril]

The recent removal of Cargo.lock (#135) made the nix flake inoperant. This fixes the flake by making he bertie package take a pointer to a Cargo.lock as argument.

[Nadrieril]

cc @karthikbhargavan

[karthikbhargavan]

Generally looks right, even though I definitely don't know enough Nix to understand it. Does the CI generate the Cargo.lock each time?

[Nadrieril]

Does the CI generate the Cargo.lock each time?

The circus-green CI does yes, and will keep the last-known-good Cargo.lock in its repo.

[msprotz]

This seems like the right thing to do to capture the idea of a "last known good" set of revisions.

We've been doing this with the everest script with good success over the years.