Enable nilnil linter for new code (#36591)

Fixes: https://github.com/go-gitea/gitea/issues/36152

Enable the `nilnil` linter while adding `//nolint` comments to existing
violations. This will ensure no new issues enter the code base while we
can fix existing issues gradually.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

services/actions/job_emitter.go

@@ -123,7 +123,7 @@ func checkJobsByRunID(ctx context.Context, runID int64) error {
 // findBlockedRunByConcurrency finds the blocked concurrent run in a repo and returns `nil, nil` when there is no blocked run.
 func findBlockedRunByConcurrency(ctx context.Context, repoID int64, concurrencyGroup string) (*actions_model.ActionRun, error) {
 	if concurrencyGroup == "" {
-		return nil, nil
+		return nil, nil //nolint:nilnil // return nil to indicate that no blocked run exists
 	}
 	cRuns, cJobs, err := actions_model.GetConcurrentRunsAndJobs(ctx, repoID, concurrencyGroup, []actions_model.Status{actions_model.StatusBlocked})
 	if err != nil {