internal: add missing checkout step to gather_prs_test.yml

[Pospelove]

[!IMPORTANT] Add missing checkout step to gather_prs_test.yml to ensure repository is checked out.

• Workflow:
  • Adds missing actions/checkout@v4 step to gather_prs_test.yml to ensure repository is checked out.
  • Configures fetch-depth: 1 and submodules: 'true' for the checkout step.

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