### Check existing issues - [x] I've verified that this bug isn't described by any existing issues. ### Bug description `ubuntu@26.04` shouldn't be a valid `build-base` before its official release, and should therefore not exist in the [schema](https://github.com/canonical/rockcraft/blob/5c56021d1f7c09ff1334d3d056a02c01b3de024d/schema/rockcraft.json#L11051). ### Steps to reproduce It exists in the schema where the current `HEAD` is tracked: https://github.com/canonical/rockcraft/blob/5c56021d1f7c09ff1334d3d056a02c01b3de024d/schema/rockcraft.json#L11043-L11063 ### Environment Irrelevant to environments. ### rockcraft.yaml ```yaml <!-- If the issue is specific to a project, share its `rockcraft.yaml`. This field automatically renders as YAML, so you don't need to add special formatting. --> ``` ### Log output ```shell <!-- Provide any relevant logging. This field automatically renders as a log, so you don't need to add special formatting. --> ```