chore: Use aws-lc nix flake from upstream

[dougch]

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Problem:

The s2n-tls nix flake is pointing at a fork, which isn't being updated regularly.

Solution:

Flipping aws-lc to point to the upstream flake fails with a build issue.

A PR to address the build failures aws/aws-lc#2431

Requirements / Acceptance Criteria:

nix build and nix develop from upstream should run successfully.

Out of scope:

Is there anything the solution will intentionally NOT address?