Get [rust-toolchain] from [opam-repository]

Context

Update the opam-repository tag to point to the new version of opam-repository!450 (merged). Opam-repository now also builds the rust-toolchain image as another tag along with the preexsting runtime* tags.

Note: the rust-toolchain image is used in the CI jobs:

• build_kernels
• test_kernels
• publish_kernel_sdk

and to build the tezos/tezos docker images. Care should be taken that these jobs still work as before.

Manually testing the MR

• Check out the kernel related jobs in the CI
• Try building the images locally: ./scripts/create_docker_image.sh -- after which, how to check that the evm kernel in there is good?

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