Fix issue will be detected as pull request when checking `First-time contributor` (#28237) (#28271)

Backport #28237 by @yp05327

Fix #28224

Co-authored-by: yp05327 <576951401@qq.com>

routers/web/repo/issue.go

@@ -1306,7 +1306,7 @@ func roleDescriptor(ctx stdCtx.Context, repo *repo_model.Repository, poster *use
 		return roleDescriptor, err
 	} else if hasMergedPR {
 		roleDescriptor.RoleInRepo = issues_model.RoleRepoContributor
-	} else {
+	} else if issue.IsPull {
 		// only display first time contributor in the first opening pull request
 		roleDescriptor.RoleInRepo = issues_model.RoleRepoFirstTimeContributor
 	}