images: use a fix version of opam instead of edge from alpine

What

This MR addresses CI/build failures caused by a recent opam release, as seen in this job failure. The root cause is that the opam package shipped with Alpine can change unexpectedly, leading to non-reproducible builds and breakages.

To solve this, we now use a script (images/scripts/install_opam_static.sh) to download and verify a static release of opam from the official repository, instead of relying on the Alpine package manager. This ensures that the CI always uses a known, tested version of opam, improving build stability and reproducibility.

Why

How

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