chore(nix): Flip awslc to upstream flake.

[dougch]

Release Summary:

Resolved issues:

none

Description of changes:

While working through getting the aws-lc flake merged, we used a branch. Now that LC has a flake on main, it's time to start using it.

Call-outs:

This will setup the nix CI job to always use mainline aws-lc.

Testing:

How is this change tested (unit tests, fuzz tests, etc.)? locally, and in CI
How can you convince your reviewers that this PR is safe and effective?
Is this a refactor change? If so, how have you proved that the intended behavior hasn't changed?

Remember:

• Any change to the library source code should at least include unit tests.
• Any change to the core stuffer or blob methods should include CBMC proofs.
• Any change to the CI or tests should:
  1. prove that the test succeeds for good input
  2. prove that the test fails for bad input (eg, a test for memory leaks fails when a memory leak is committed)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.