fix(nix): use cargoTestFlags instead of doCheck = false 🔧

[Klazomenai]

Summary

• Replace doCheck = false with cargoTestFlags = [ "--lib" "--test" "integration" ]
• Nix builds now run unit + integration tests in the sandbox, only skipping E2E tests that require Foundry artifacts (contracts/out/)
• PR fix(nix): skip test compilation in Nix build 🔧 #33 merged the original doCheck = false before this fix could be pushed

Refs #9

Test plan

• nix build .#alpha succeeds with tests running
• CI passes (unit + integration tests)

[Copilot]

Pull request overview

Updates the Nix build to run Rust unit + integration tests in the sandbox while still avoiding E2E tests that depend on Foundry artifacts (contracts/out/), improving build-time confidence without reintroducing the prior sandbox failure.

Changes:

• Replace doCheck = false with cargoTestFlags to run cargo test --lib and cargo test --test integration during Nix builds.
• Update the inline comment to describe why E2E tests are excluded from Nix builds.

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[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

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