ci: add concurrency group to functions.yaml to cancel redundant runs

[Ankitsinghsisodya]

Summary

• Adds a concurrency block to .github/workflows/functions.yaml
• Without it, pushing multiple commits to the same PR (or re-running after a fixup) queues multiple workflow runs simultaneously, wasting runner minutes on results that will be superseded
• With cancel-in-progress: true, any in-progress run for the same branch or PR ref is cancelled when a new run starts

Test plan

• Push two commits to a PR branch in quick succession and confirm the first run is cancelled when the second starts
• Confirm pushes to main still run to completion (each push to main gets its own group key)

Fixes #3531

[knative-prow]

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[linux-foundation-easycla]

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• ✅ login: Ankitsinghsisodya / name: Ankit sisodya (52b1216, f9ebdf5)

[knative-prow]

Hi @Ankitsinghsisodya. Thanks for your PR.

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[Copilot]

Pull request overview

Adds GitHub Actions workflow concurrency controls to reduce redundant CI runs for the Functions workflow.

Changes:

• Introduces a concurrency block in .github/workflows/functions.yaml to cancel in-progress runs when a new run starts for the same group key.

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[Copilot]

The PR description/test plan says pushes to main should run to completion with a unique concurrency group per push, but group: ${{ github.workflow }}-${{ github.ref }} will use refs/heads/main for all main pushes, so a newer push can cancel an in-progress main run. If the intent is to only cancel redundant PR runs, consider making cancel-in-progress conditional (e.g., only for pull_request events) and/or adjusting the group key to differentiate main pushes.

[Ankitsinghsisodya]

Good catch. Pushes to main trigger build and publish jobs — cancelling those mid-flight would be dangerous. Fixed in f9ebdf5 by making cancel-in-progress conditional:

cancel-in-progress: ${{ github.event_name == 'pull_request' }}

Now only redundant PR runs are cancelled; main pushes always run to completion.