chore: add nix builds and caching

[tgunnoe]

Description

We're currently using the nix devshells locally and in the CI, but with a raw cargo caching solution on our infra. This PR builds our substrate project using https://github.com/ipetkov/crane, a minimal nix-based wrapper that also excels at only rebuilding direct dependencies and not rebuilding upstream deps like the substrate project unless Cargo.lock changes for it. In the process it simplifies the caching solution and LOC required by using nix for caching, a task at which it already excels at.

This PR keeps the current devshell workflow and does not modify it. This PR does not modify the normal CI workflow as of yet-- instead for now it adds a new github action workflow for testing purposes.

Tested on macOS and Linux for building with cargo directly.

https://input-output.atlassian.net/browse/ETCM-12378?atlOrigin=eyJpIjoiNGZhOGYzMzdkZjEwNGU5NmI3MzMxODZjMDYzOGRmZmQiLCJwIjoiaiJ9

Checklist

• Commit sequence broadly makes sense and commits have useful messages.
• The size limit of 400 LOC isn't needlessly exceeded
• The PR refers to a JIRA ticket (if one exists)
• New tests are added if needed and existing tests are updated.
• New code is documented and existing documentation is updated.
• Relevant logging and metrics added
• Any changes are noted in the changelog.md for affected crate
• Self-reviewed the diff

[skylar-simoncelli]

Approved, though I don't entirely understand why we need to merge a second CI workflow into master.

If your tests of Attic are working then can go straight to modifying the main ci.yml workflow and testing in a branch

[kpinter-iohk]

Tested nix dev shell with my workflows, looking good! 🧑‍🍳👌

[tgunnoe]

If your tests of Attic are working then can go straight to modifying the main ci.yml workflow and testing in a branch

We can use it on the single CI job. I'll need to test it first on this branch with the rest of the workflow. thanks