CI: use [GCP_PUBLIC_REGISTRY] to locate the opam-repo images

What

Introduce and use the variable GCP_PUBLIC_REGISTRY to refer to the public GCP docker registry, and to locate the tezos/opam-repository images.

Why

In !12970 (merged), we modified the variable GCP_REGISTRY such that it points to a protected GCP registry on protected branches, and to a public GCP registry on non-protected branches. However, the same variable is used to locate the opam-repository images. But, these images are always pushed to the public GCP registry. Consequently, we're now looking for these images in the wrong location, like seen here: https://gitlab.com/tezos/tezos/-/jobs/6716033882

We've temporarily circumvented this by changing the value of GCP_REGISTRY so that it always points to the public registry. Once this MR is merged, we can revert to the previous state.

How

Just using the new variable. The new variable has already been added in the GitLab settings.

Manually testing the MR

If the variable is present, then this pipeline will pass.

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