ci: [lint] Use Cirrus dockerfile cache #31948

The lint task is problematic, because:

• It doesn't check modifications to ci/lint_imagefile
• It calls a separate script that installs packages on every run (taking time)
• It uses *_cache instructions to cache some installed parts, but not all

Fix all issues by using ci/lint_imagefile (https://cirrus-ci.org/guide/docker-builder-vm/#dockerfile-as-a-ci-environment)

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/31948.

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #31787 (ci: run in worktrees by willcl-ark)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

cc @willcl-ark

ACK fa3b4427158d48f7d899582580f8f6a7b1bc981d

Sensible change to speedup the lint task startup.

The change looks correct, and to me it looks like it also works, judging by the log lines:

11:51:41 PM Successfully pulled image "gcr.io/cirrus-ci-community/bitcoin/bitcoin/ci/lint_imagefile:714fc33ccc0872eef4aa515b2c226494648ec0f52aa6d3afbaf779c6d1a976b3" in 17.583s (18.557s including waiting). Image size: 699506090 bytes.

...and from the "dependency run" step, which is building an image for this imagefile: https://cirrus-ci.com/task/5351025877450752

I do notice the linked doc page claims that:

Under the hood, for every Dockerfile that is needed to be built, Cirrus CI will create a Docker Builder task as a dependency. You will see such build_docker_image_HASH tasks in the UI.

And I wasn't able to see any of those lines anywhere in the UI; but perhaps that's changed since it was documented, or they don't appear if the image is already cached.