Release workflow: unnecessary macOS build and duplicate CI after merge

[lucaromagnoli]

Problem

• After merging a release PR, the release workflow is building macOS, but we do not need macOS builds for releases.
• After merging, the CI build-and-test workflow is running instead of (or in addition to) the release workflow. This is redundant and wastes GitHub Actions minutes.

Desired behavior

• Only build Linux and Windows in the release workflow (no macOS).
• After merging a release PR, only the release workflow should run, not CI build-and-test.

Steps to reproduce

1. Merge a release PR to main
2. Observe that macOS is built and CI build-and-test runs

Acceptance criteria

• Release workflow does not build macOS
• CI build-and-test does not run after merging a release PR
• Only the release workflow runs on tag push after merge

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This will save costs and reduce redundant builds. See recent merges for examples.