ci(subscriptions): point channels workflow testcontainers at remote Docker host

[aacruzgon]

Summary

Fixes #389. The subscriptions-channels.yml workflow fails on the current [self-hosted, Linux] runner pool: the runners no longer expose a local /var/run/docker.sock, so every testcontainer-based test in the unit-tests matrix panics with SocketNotFoundError — and the email integration tests silently skip via their docker_available() guard, reporting green without testing anything. subscriptions-smoke.yml already adapted by pointing testcontainers at a remote Docker host via DOCKER_HOST; the channels workflow never got that env block.

Changes

• .github/workflows/subscriptions-channels.yml:
  • add the DOCKER_HOST: ${{ secrets.DOCKER_HOST }} / DOCKER_HOST_IP env block used by subscriptions-smoke.yml, with a comment referencing subscriptions-channels.yml fails on self-hosted Linux runners: no docker.sock (needs DOCKER_HOST env like smoke workflow) #389;
  • run the unit-test step with --no-fail-fast, so a failing test binary no longer aborts cargo test before the remaining suites (previously the persistence failure masked that the subscriptions test binaries never ran at all).

Testing

• Validated by dispatching the workflow with this exact change from feat/cluster-capable-state (run 30099280985): all 112 jobs green.
• The Email matrix jobs' mailpit tests genuinely ran — 5 passed in 1.94s, versus the prior 0.01s silent skip — because docker_available() also honors DOCKER_HOST, so this change simultaneously un-breaks the unit tests and reactivates real email delivery coverage.

Notes

• The validation commit is being reverted out of feat/cluster-capable-state (feat(cluster): make HFS cluster-capable behind a load balancer #269) so this lands independently; feat(cluster): make HFS cluster-capable behind a load balancer #269 will pick it up on its next merge from main.
• Follow-up hardening (fail-loudly-in-CI guards instead of silent skips) is tracked in subscriptions-channels.yml fails on self-hosted Linux runners: no docker.sock (needs DOCKER_HOST env like smoke workflow) #389/Test suites can pass vacuously: env-gated tests silently skip in CI (sibling of #389) #390 ("PR B").

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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