AWS-hosted runners

There are multiple ways to add AWS-hosted runners to boostorg/boost-ci.

1. Update .github/workflows/ci.yml so that it uses the new runners instead of GitHub runners.

or

2. Add another file .github/workflows/ci-aws-hosted.yml This offers a choice, and doesn't force the new method. In the short term this is probably the way to go, because the upstream terraform project is in flux.

or

3. An open pull request "feat: Adding runner_enable_default_labels variable #3491" in https://github.com/philips-labs/terraform-aws-github-runner should be included in the design, and when it's available that will change the instructions for boost libraries. Therefore, wait before continuing.

In the meantime anyone is welcome to experiment with the self-hosted runners https://github.com/cppalliance/githubactions however the config will need to be modified.