fix(workflow): update ISO workflow regarding uploads

[inffy]

Small changes to uploads

• During PR checks -> uploads job artifacts
• Merge Queu merge -> uploads ISO to r2 testing bucket and no job artifacts

Other things are kept, like the manual workflow dispatch etc

[Copilot]

Pull request overview

Updates the ISO build workflows so PR runs upload build outputs as GitHub job artifacts, while merge queue (merge_group) runs upload the ISO outputs to the Cloudflare R2 testing bucket without producing job artifacts.

Changes:

• Switch artifact uploading in the reusable workflow to be controlled by inputs.upload_artifacts (instead of event-name checks).
• Switch R2 upload steps in the reusable workflow to be controlled by inputs.upload_r2.
• Update the stable ISO caller workflow to set upload_artifacts for PR checks and set upload_r2 for merge queue + schedule + manual dispatch.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
.github/workflows/reusable-build-iso-anaconda.yml	Gates artifact and R2 upload steps solely via reusable-workflow inputs.
.github/workflows/build-iso-stable.yml	Sets reusable-workflow inputs based on pull_request, merge_group, schedule, and workflow_dispatch triggers to match the desired upload behavior.

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