Also upload the artifact to GitHub action, and in addition use the same setup
(ubuntu 20.04 image) and build directories as done on builds.robur.coop.
Also use `strip` on the resulting binary to reduce it's size (since the debug
section aren't mapped into the running unikernel, there's nothing we get from
them -- also they are preserved (as .debug file) and uploaded to
https://builds.robur.coop if one needs them).
This entails binary reproducibility between the different systems:
- a developer using ./build-with-docker.sh
- GitHub action (run on every PR)
- builds.robur.coop with the ubuntu-20.04 worker
