fix: cache-from/cache-to need 'ref=' prefix

Description

When running ./grove run-pipeline build-instance-image I got this error:

ERROR: invalid value registry.gitlab.com/opencraft/ops/grove-production-digitalocean/clipboard-preview/openedx:build-cache

From the docs, it seems to be missing the ref= prefix.

I bumped TOOLS_CONTAINER_IMAGE_VERSION along with this fix, since the change happens within the tools container.

Supporting information

The version in openedx.yml was already correct.

Testing instructions

Run ./grove run-pipeline build-instance-image for some instance.

Dependencies

None

Screenshots

n/a

Checklist

If any of the items below is not applicable, do not remove them, but put a check in it.

• All providers include the new feature/change
• All affected providers can provision new clusters
• Unit tests are added/updated
• Documentation is added/updated
• The TOOLS_CONTAINER_IMAGE_VERSION in ci_vars.yml is updated
• The grove-template repository is updated

Additional context

Ticket: MNG-3815