CI: instantiate opam environment in [oc.unified_coverage] on [master]

What

Runs eval $(opam env) in the oc.unified_coverage job running on the master branch.

Why

Fixes the issue /scripts-3836952-6897726662/step_script: line 210: dune: command not found as reported e.g. here.

Manually testing the MR

Simulated pipeline: https://gitlab.com/tezos/tezos/-/pipelines/1298798227

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