ci: use GitHub-hosted runner to upload sources

In the `.github/workflows/source.yml` workflow for preparing a tarball
with the source code, a PackPack Docker container is already used. For
uploading the tarball to the repo, the `aws` utility is used, which is
installed before. To skip installation of additional packages on the
self-hosted runners, we are moving to the GitHub-hosted runners, which
already have the `aws` utility installed. Step `Prepare checkout` is
removed because the GitHub-hosted runner is an ephemeral environment.

NO_DOC=ci
NO_TEST=ci
NO_CHANGELOG=ci