fix: guard SetJobError against missing job-error container (#1050)

Fixes https://gitea.com/gitea/runner/issues/1047

The runner panics during shutdown when jobs are interrupted:

```
panic in executor: interface conversion: interface {} is nil, not map[string]error
...
gitea.com/gitea/runner/act/common.SetJobError(...)
    /data/gitea/runner/act/common/job_error.go:27
gitea.com/gitea/runner/act/runner.reportStepError(...)
    /data/gitea/runner/act/runner/job_executor.go:62
```

`SetJobError` did an unchecked type assertion on the job-error container stored in the context:

```go
ctx.Value(jobErrorContextKeyVal).(map[string]error)["error"] = err
```

When the container is absent — e.g. on shutdown, when a job is interrupted and `reportStepError` → `SetJobError` runs on a context where `WithJobErrorContainer` was never called — `ctx.Value` returns `nil` and asserting `nil.(map[string]error)` panics.

## Fix

Use the comma-ok form so a missing container is a no-op, matching the safe pattern already used in the sibling `JobError` function. If there's no container, there's nowhere to record the error anyway, so skipping is correct.

```go
func SetJobError(ctx context.Context, err error) {
	if container, ok := ctx.Value(jobErrorContextKeyVal).(map[string]error); ok {
		container["error"] = err
	}
}
```

Reviewed-on: https://gitea.com/gitea/runner/pulls/1050
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: bircni <bircni@icloud.com>
Co-committed-by: bircni <bircni@icloud.com>

act/common/job_error.go

@@ -24,7 +24,9 @@ func JobError(ctx context.Context) error {
 }
 
 func SetJobError(ctx context.Context, err error) {
-	ctx.Value(jobErrorContextKeyVal).(map[string]error)["error"] = err
+	if container, ok := ctx.Value(jobErrorContextKeyVal).(map[string]error); ok {
+		container["error"] = err
+	}
 }
 
 // WithJobErrorContainer adds a value to the context as a container for an error