fix: new issue workflow (flashinfer-ai#1773)

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## 📌 Description

The intent of the workflow is to add `needs-triage` when a new issue is
created. The initial version of this workflow, added in flashinfer-ai#1765 failed
when I created dummy issue flashinfer-ai#1772. This PR is a fix for that bug.

## 🔍 Related Issues

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## 🧪 Tests

I have tested with `gh` CLI targeting my fork of this repo and also by
adding the action to my fork.

Completed action:
https://github.com/sricketts/flashinfer/actions/runs/18041739971

- [x] Tests have been added or updated as needed.
- [ ] All tests are passing (`unittest`, etc.).

## Reviewer Notes

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Author: sricketts

.github/workflows/new-issue.yml

@@ -1,35 +1,20 @@
-name: Triage new issues
+name: Triage new issue
 
 on:
-  issues:
-    types: [opened]
-
-permissions:
-  issues: write
+    issues:
+      types:
+        - opened
 
 jobs:
-  triage:
+  label_issues:
     runs-on: ubuntu-latest
+    permissions:
+      issues: write
     steps:
       - name: Add needs-triage label
-        uses: actions/github-script@v7
-        with:
-          github-token: ${{ secrets.GITHUB_TOKEN }}
-          script: |
-            const issueNumber = context.issue.number;
-            const { owner, repo } = context.repo;
-            const labelName = 'needs-triage';
-            try {
-              await github.rest.repos.getLabel({ owner, repo, name: labelName });
-            } catch (error) {
-              if (error.status === 404) {
-                throw new Error(`Required label '${labelName}' does not exist in ${owner}/${repo}. Please create it in the repository settings.`);
-              }
-              throw error;
-            }
-            await github.rest.issues.addLabels({
-              owner,
-              repo,
-              issue_number: issueNumber,
-              labels: [labelName],
-            });
+        run: gh issue edit "$NUMBER" --add-label "$LABELS"
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          GH_REPO: ${{ github.repository }}
+          NUMBER: ${{ github.event.issue.number }}
+          LABELS: needs-triage