ci: disable builtin continual tasks

[teskje]

This PR disables the builtin continual tasks in CI.

Context in Slack.

Motivation

• This PR adjusts a CI config.

The motivation is twofold:

• We are not enabling builtin CTs anywhere else by default (prod, staging, local dev) and nobody is currently driving the feature forward. Disabling the builtin CTs in CI makes our CI config more similar to what we run in production.
• We don't have migration implemented for builtin CTs, and the lack of migration support blocks PRs that want to change builtin sources that have CTs built on them (such as controller: emit undefined lag for unreadable collections #32329).

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.

[teskje]

I've left a note about the lack of migration support for CTs in the CT epic, in case we'll pick this up again at some point.

[def-]

Nightly triggered: https://buildkite.com/materialize/nightly/builds/12090 Edit: Lgtm, or at least unrelated to this PR

[bkirwi]

Makes sense!

[teskje]

TFTRs!