Add concurrency policy to e2e.yml workflow to prevent overlapping runs

[Copilot]

This PR adds a concurrency policy to the e2e.yml workflow to automatically cancel in-progress runs when new ones are triggered, preventing resource conflicts and ensuring efficient CI usage.

Problem

The e2e workflow can be triggered by multiple sources:

• Nightly schedule (3:00 AM UTC)
• Manual workflow dispatch
• Changes to the workflow file itself

Without concurrency control, multiple instances can run simultaneously, potentially causing:

• Resource conflicts between test runs
• Interference with test artifacts and GitHub API operations
• Wasted CI resources on redundant runs

Solution

Added a concurrency configuration that uses the workflow name as the grouping key:

concurrency:
  group: ${{ github.workflow }}
  cancel-in-progress: true

This ensures that:

• Only one instance of the "Nightly E2E Tests" workflow runs at a time
• Any in-progress run is automatically cancelled when a new one starts
• The most recent trigger always takes precedence
• No git reference is included in the grouping key, so all triggers are treated equally

Benefits

• Prevents conflicts: Eliminates interference between concurrent test runs
• Saves resources: Avoids running redundant workflows
• Ensures freshness: Always tests the latest changes when multiple triggers occur
• Maintains reliability: Reduces flaky test failures caused by resource contention

The change is minimal and surgical, adding only 4 lines to the existing workflow without affecting any other functionality.

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