ci: run integration tests with the remote multi-node worker

[crazy-max]

follow-up #3716 (comment)

This adds a dedicated remote+multinode integration worker backed by the docker test environment. The new worker provisions two docker-container builders for different platforms and exposes them as a single appended remote builder. This gives the integration suite a stable multi-node topology.

[crazy-max]

Following tests fail with multi-node:

 === Failed
=== FAIL: tests TestIntegration/TestHistoryBuildName/worker=remote+multinode/bake_git (0.44s)
history.go:291: Error Trace:	/src/tests/history.go:291
Error: Received unexpected error:
exit status 1
Test: TestIntegration/TestHistoryBuildName/worker=remote+multinode/bake_git
--- FAIL: TestIntegration/TestHistoryBuildName/worker=remote+multinode/bake_git (0.44s)
=== FAIL: tests TestIntegration/TestHistoryBuildName/worker=remote+multinode (2.30s)
--- FAIL: TestIntegration/TestHistoryBuildName/worker=remote+multinode (2.30s)
=== FAIL: tests TestIntegration/TestHistoryLs/worker=remote+multinode (1.50s)
history.go:85: Error Trace:	/src/tests/history.go:85
/src/vendor/github.com/moby/buildkit/util/testutil/integration/run.go:103
/src/vendor/github.com/moby/buildkit/util/testutil/integration/run.go:251
Error: Received unexpected error:
exit status 1
Test: TestIntegration/TestHistoryLs/worker=remote+multinode
--- FAIL: TestIntegration/TestHistoryLs/worker=remote+multinode (1.50s)

Slightly relates to #3716, skipping for now.