CI: pin GitHub Actions runners

[ErikSchierboom]

This PR updates GitHub Actions runners to a specific version.
This ensures that the workflow will always run on the same runner, which makes your build stable.

The PR updates the *-latest version with the current version, as specified in https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners/about-github-hosted-runners#supported-runners-and-hardware-resources.

See https://exercism.org/docs/building/github/gha-best-practices#h-pin-test-runners-to-version for more information.

[github-actions]

Thank you for contributing to exercism/elixir 💜 🎉. This is an automated PR comment 🤖 for the maintainers of this repository that helps with the PR review process. You can safely ignore it and wait for a maintainer to review your changes.

Based on the files changed in this PR, it would be good to pay attention to the following details when reviewing the PR:

• General steps
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