Proto-env: prepare for zarith bump to 1.13

Context

Zarith.1.3 changed extract from an external to a regular val.

We need this change before we can bump zarith. The plan is to remove the problematic extract external expose in env [1 .. 10] later after there are all no longer active. It's not used in any of the of protocols.

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR