fix(contract build): use `--locked` when building contracts to ensure deps remain the same with the same `Cargo.lock`

[willemneal]

What problem does your feature solve?

Improving contract build determinism so that deps are the same as long as the Cargo.lock remains the same.

This way anyone who wants to rebuild the contract in the future will use the pinned versions instead of potentially pulling in updated deps.

What would you like to see?

What alternatives are there?

[willemneal]

fixed by #2383