Add Linux dev build group to Buildkite pipeline

[ivan-ottinger]

Related issues

Part of RSM-1314 — Linux release prep.
Apps Infra request: AINFRA-2352.

How AI was used in this PR

AI-assisted: drafted the new pipeline group by mirroring the existing Mac and Windows groups, and drafted this PR body. Author reviewed.

Proposed Changes

Adds a 📦 Build for Linux group to .buildkite/pipeline.yml, mirroring the existing Mac and Windows groups:

• Manual 🚦 Build for Linux? input gate, skipped on tags and trunk (same pattern as Mac/Windows).
• Matrix [x64, arm64] on queue: linux.
• Runs npm run make:linux-{{matrix}} (scripts added in Add Linux make scripts for cross-architecture builds #3326).
• Uploads the resulting .deb files as artifacts.

Intentionally not included:

• prepare-environment.sh — it installs Fastlane gems and signing certs (install_gems, set_up_signing) that the unsigned Linux DEBs don't need.
• Distribute Dev Builds integration — separate follow-up. Requires adding Linux entries to the existing distribute_builds method in fastlane/Fastfile and updating the WPCOM AppsCDN side to accept Linux - x64 / Linux - ARM64 as platform values (see RSM-1315).

Testing Instructions

This PR cannot be exercised end-to-end until a Linux Buildkite agent is provisioned and registered to the linux queue (AINFRA-2352). Once that lands:

1. Push a commit to the branch — the input gate appears on the PR.
2. Click 🚦 Build for Linux? — both matrix jobs run.
3. Verify the x64 and arm64 jobs each upload a .deb artifact.

Depends on #3326 (the make:linux-* scripts). Merge that first.

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[Copilot]

Pull request overview

Adds a new optional Linux dev-build group to the Buildkite pipeline so Studio can produce Linux .deb artifacts alongside the existing Mac and Windows dev builds.

Changes:

• Adds a manual Build for Linux? input gate for non-tag, non-trunk builds.
• Adds a 📦 Build for Linux group with x64 and arm64 matrix jobs on the linux queue.
• Runs Linux packaging commands and uploads generated .deb artifacts.

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[gavande1]

LGTM ✅ Left a minor inline note about the dev-build distribution gap.

[gavande1]

Nit (non-blocking): Could you add a short comment in this group pointing to RSM-1315? Something like:

  - group: 📦 Build for Linux
    key: dev-linux
    depends_on: input-dev-linux
    if: build.tag !~ /^v[0-9]+/
    # Note: Linux DEBs are produced here but not yet picked up by the
    # "Distribute Dev Builds" step below — that integration is tracked in RSM-1315.
    steps:

The PR body already explains this, but a reader looking at the YAML in six months won't have the PR context.