ci: bump buildkit to v0.31.0

[cpuguy83]

Pins the CI builders to moby/buildkit:v0.31.0 and bumps the buildkit Go module to match so both the integration (distro) matrix and the e2e suite build against the released buildkitd.

Changes

• Add a top-level BUILDKIT_TEST_IMAGE env as the single source of truth for the buildkit image under test.
• integration job: create and --use a buildx container builder pinned to the test image instead of relying on the engine's embedded buildkit. The test harness (buildx dial-stdio) and the bake frontend/bake worker pre-builds all target this builder. The failure log dump now also collects the buildx_buildkit_* container logs.
• e2e job: create a pinned container builder for both diff/merge matrix legs (USE_BUILDX=1 for both), rather than only the diff/merge-enabled leg using a container builder and the other falling back to the default docker driver.
• Bump github.com/moby/buildkit v0.30.0 -> v0.31.0 (with the transitive updates from go mod tidy).

Trade-offs / notes

• The integration job previously used the default docker driver, so bake pre-builds landed in the docker image store. With a container builder they warm the builder cache instead; tests consume the frontend as a buildkit frontend-input and reuse the same builder, so this is functionally equivalent but is the main thing to watch on the first run.

Local validation: go build ./..., go run ./cmd/lint ./..., go generate ./... (no diff), go test --test.short ./... all pass.

[Copilot]

Pull request overview

This PR temporarily pins the CI environment to BuildKit v0.31.0-rc2 for validation. It introduces a workflow-level BUILDKIT_TEST_IMAGE env var as the single source of truth for the buildkit image, creates explicit docker buildx container builders in both the integration and e2e jobs (instead of relying on the engine's embedded buildkit), and bumps the github.com/moby/buildkit Go module from v0.30.0 to v0.31.0-rc2 along with all transitive dependency updates.

Changes:

• Add BUILDKIT_TEST_IMAGE env var and create pinned container builders in both the integration and e2e CI jobs so all test suites exercise the RC buildkitd.
• Improve failure-mode log collection in the integration job to capture both dockerd and buildx container builder logs.
• Bump github.com/moby/buildkit to v0.31.0-rc2 in go.mod/go.sum with transitive dependency updates from go mod tidy.

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated no comments.

File	Description
.github/workflows/ci.yml	Adds workflow-level BUILDKIT_TEST_IMAGE env, creates pinned buildx builders in integration and e2e jobs, improves log dump to capture buildx container logs
go.mod	Bumps buildkit to v0.31.0-rc2, Go version to 1.25.9, removes deprecated golang/protobuf indirect dep, updates transitive dependencies
go.sum	Corresponding checksum updates for all bumped transitive dependencies