This removes all CI builds from travis-ci, due to their recent changes
in policy and harsh limitations on builds. With build times over 2
hours, it was a significant hindrance to development.
Instead of Travis CI, the CI is now split on Sourcehut and GitHub. Since
Sourcehut only supports Linux/BSD, all builds on those operating systems
are executed there. The GitHub Actions CI is used to build for
Windows/macOS, which are not available on Sourcehut.
Since asset deployment for releases requires builds on all platforms,
this is also done on GitHub actions. Though the new `upload_asset.sh`
script makes sure that migration in the future is fairly simple and we
do not tie ourselves to the overly complicated GitHub Actions ecosystem.
Christian Duerr