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>

routers/web/shared/actions/runners.go

@@ -59,7 +59,7 @@ func getRunnersCtx(ctx *context.Context) (*runnersCtx, error) {
 	if ctx.Data["PageIsOrgSettings"] == true {
 		if _, err := shared_user.RenderUserOrgHeader(ctx); err != nil {
 			ctx.ServerError("RenderUserOrgHeader", err)
-			return nil, nil
+			return nil, nil //nolint:nilnil // error is already handled by ctx.ServerError
 		}
 		return &runnersCtx{
 			RepoID:             0,

routers/web/shared/actions/variables.go

@@ -51,7 +51,7 @@ func getVariablesCtx(ctx *context.Context) (*variablesCtx, error) {
 	if ctx.Data["PageIsOrgSettings"] == true {
 		if _, err := shared_user.RenderUserOrgHeader(ctx); err != nil {
 			ctx.ServerError("RenderUserOrgHeader", err)
-			return nil, nil
+			return nil, nil //nolint:nilnil // error is already handled by ctx.ServerError
 		}
 		return &variablesCtx{
 			OwnerID:           ctx.ContextUser.ID,