Skip set-rn-artifacts-version for PRs targeting stable branches (#55809)

Summary:
When a PR targets a `-stable` branch, `github.ref_name` is the PR branch (not ending in `-stable`), so the version script was incorrectly running. Added a check for `github.base_ref` to also skip the script when the PR target branch is a stable branch.

## Changelog:
[Internal] - Skip set-rn-artifacts-version for PRs targeting stable branches

Pull Request resolved: https://github.com/facebook/react-native/pull/55809

Test Plan: CI should pass without running the artifacts version script on PRs targeting stable branches.

Reviewed By: cortinico

Differential Revision: D94678295

Pulled By: cipolleschi

fbshipit-source-id: 5809ec710c191075f1bc1528ecff95f8bd745257