fix(acir): add bn254 as default feature flag

[TomAFrench]

Related issue(s)

Resolves (CI failure)

Description

Summary of changes

We currently can't publish releases as cargo publish builds each package before publication and acir doesn't build without a feature flag being set (this wasn't encountered in real usage as we always set this flag when building in the context of the workspace/noir).

See: https://github.com/noir-lang/acvm/actions/runs/4830288705/jobs/8606331047

This PR sets a default feature flag so that cargo publish can build the acir crate in isolation.

Dependency additions / changes

(If applicable.)

Test additions / changes

(If applicable.)

Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt with default settings.
• I have linked this PR to the issue(s) that it resolves.
• I have reviewed the changes on GitHub, line by line.
• I have ensured all changes are covered in the description.

Additional context

(If applicable.)