.github: Revived remote cache testing

This patch does the following things:

  * ci.remote-execution-cluster.yml -> ci.remote-execution.yml

  * .github/workflows/ci.yml: Refactored to add a second test matrix

    The 'test-services' matrix runs tests which require adjacent services
    run in parallel with the test defined in ci.docker-compose.yml

  * .github/compose/ci.buildstream-remote-cache.yml: Added composition
    file to run the buildstream artifact cache `bst-artifact-server`

  * .github/compose/ci.buildbarn-remote-cache.yml: Added composition
    file to run the buildbarn index and storage services.

  * .github/compose/ci.docker-compose.yml: Add the test declarations
    to run tests with `--remote-cache` to test against the artifact
    cache servers.
7 files changed