feat: show run command, shell and env in collapsible group before step output (#847)

## Summary

Mirrors the GitHub Actions runner behaviour where each `run:` step shows a collapsible **"Run \<command\>"** section containing the script, shell command, and environment variables before the actual step output.

### What changes

- **`pkg/runner/step_run.go`**: In `stepRun.main()`, two new executors are added to the pipeline:
  1. `logRunGroupHeader()` — runs after `setupShellCommandExecutor()` (so `sr.cmdline` is already resolved). Emits a `::group::Run <step>` log entry followed by the interpolated script, the full shell command line, and the step's env vars (sorted, internal vars filtered out).
  2. The existing execution function now has `defer rawLogger.Infof("::endgroup::")` so the group is closed after the step finishes, regardless of success or failure.

### Env var filtering

Internal runner vars are hidden (`GITHUB_*`, `GITEA_*`, `RUNNER_*`, `INPUT_*`, `PATH`, `HOME`) — only user-relevant vars are shown, matching what GitHub Actions displays.

### Example output

```
▼ Run cargo build
  cargo build
  shell: bash --noprofile --norc -e -o pipefail {0}
  env:
    CARGO_HOME: /home/runner/.cargo
    CARGO_INCREMENTAL: 0
    CARGO_TERM_COLOR: always
  <actual build output>
```

---------

Co-authored-by: silverwind <2021+silverwind@noreply.gitea.com>
Co-authored-by: silverwind <me@silverwind.io>
Reviewed-on: https://gitea.com/gitea/act_runner/pulls/847
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>
Reviewed-by: ChristopherHX <38043+christopherhx@noreply.gitea.com>

act/runner/step_action_remote.go

@@ -166,6 +166,10 @@ func (sar *stepActionRemote) main() common.Executor {
 	return common.NewPipelineExecutor(
 		sar.prepareActionExecutor(),
 		runStepExecutor(sar, stepStageMain, func(ctx context.Context) error {
+			printRunActionHeader(ctx, sar.Step, sar.env, sar.RunContext)
+			rawLogger := common.Logger(ctx).WithField(rawOutputField, true)
+			defer rawLogger.Infof("::endgroup::")
+
 			github := sar.getGithubContext(ctx)
 			if sar.remoteAction.IsCheckout() && isLocalCheckout(github, sar.Step) && !sar.RunContext.Config.NoSkipCheckout {
 				if sar.RunContext.Config.BindWorkdir {