Patch set-output deprecation in workflows

[matiboux]

Changes

I've noticed some workflows are showing warning about set-output deprecation.

This PR updates all workflows that are using set-output to use the new environment files instead.

This has been done following GitHub's "GitHub Actions: Deprecating save-state and set-output commands" blog post and instructions on updating.

Testing

I've run syntax checks on the updated workflow files.
Their behavior should not change.

The check-merge.yml workflow should run on this PR, succeed, and the step "Check if any changesets contain minor changes" should not show any warning anymore.

The main.yml and release.yml workflows won't run on this PR. I don't know how to rigorously test them further after syntax checks.

Docs

This only fixes a deprecation warning.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 05eee1e

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[bluwy]

LGTM. I'm not a bash expert, so hoping to get another eye on this before merging.

[natemoo-re]

This looks great, I had been meaning to clean this up for a while. Thanks so much for contributing!