Move the coverage-linux workflow to separate group

[jasnell]

Moves it to a separate group to avoid it making the main test workflow take much longer to complete.

[ask-bonk]

ResolveMessage: Cannot find module '@opencode-ai/plugin' from '/home/runner/work/workerd/workerd/.opencode/tools/bazel-deps.ts'

github run

[ask-bonk]

@jasnell Bonk workflow failed. Check the logs for details.

View workflow run · To retry, trigger Bonk again.

[fhanau]

In terms of the motivation for this PR – the coverage job is not required to pass and no other jobs depend on it, so this wouldn't affect the time at which jobs finish or how long it takes for CI to pass. I guess one possible upside is that notifications about the test group failing will be delivered earlier?
Edit: As mentioned via chat, this would allow restarting the test group earlier. That makes sense, it would not be needed if running jobs can just be canceled when coverage is taking too long but IIRC that wasn't always possible.

[ask-bonk]

ResolveMessage: Cannot find module '@opencode-ai/plugin' from '/home/runner/work/workerd/workerd/.opencode/tools/bazel-deps.ts'

github run

[jasnell]

the main motivation is not blocking the ability to restart the main set of tests

[anonrig]

fyi: the changes in coverage.yml resulted in more workflow runs to be executed and creating congestion... several runs are now queued