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.