github actions: enable for branches

[v1v]

What does this PR do?

Enable GitHub actions on MacOS for merge commits for the main and release branches.

Why is it important?

Ensure the CI validates merges on branches.

[v1v]

7.1 does not match any existing branch

[elasticmachine]

💚 Build Succeeded

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• Start Time: 2022-03-28T14:54:37.230+0000

• Duration: 19 min 36 sec

🤖 GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

• /package : Generate the packages.

• run integration tests : Run the Elastic Agent Integration tests.

• run end-to-end tests : Generate the packages and run the E2E Tests.

• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)