fix: Add checkout step before using local action in update-locales workflow (#5938)

## Problem
The `update-locales` workflow was failing with the error:
```
Can't find 'action.yml', 'action.yaml' or 'Dockerfile' under '/home/runner/work/ComfyUI_frontend/ComfyUI_frontend/.github/actions/setup-frontend'. 
Did you forget to run actions/checkout before running your local action?
```

Ref:
https://github.com/Comfy-Org/ComfyUI_frontend/actions/runs/18270266173/job/52011427608

## Solution
Added a checkout step using `actions/checkout@v5` before the "Setup
Frontend" step. This ensures the repository code (including the local
action definition) is available before GitHub Actions tries to use it.

## Changes
- Added checkout step to `.github/workflows/update-locales.yaml`
- Uses `actions/checkout@v5` to checkout the repository before
referencing the local custom action

This is a minimal fix that follows GitHub Actions best practices.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-5938-fix-Add-checkout-step-before-using-local-action-in-update-locales-workflow-2846d73d365081cfb720ffaa528ce26e)
by [Unito](https://www.unito.io)

---------

Co-authored-by: github-actions <[email protected]>

Author: 2 people

.github/workflows/update-locales.yaml

@@ -14,6 +14,9 @@ jobs:
     if: github.event_name == 'workflow_dispatch' || (github.event.pull_request.head.repo.full_name == github.repository && startsWith(github.head_ref, 'version-bump-'))
     runs-on: ubuntu-latest
     steps:
+    - name: Checkout repository
+      uses: actions/checkout@v5
+    
     - name: Setup Frontend
       uses: ./.github/actions/setup-frontend