gpgv: can't allocate lock for '/home/runner/.gnupg/aptlytest.gpg'
this forced running local system tessts in /home/runner, as it is
in the gitgub actions.
- use s3 mirror instead of internet download
- reduce download verbosity
- do not use venv in docker-system-tests
- be more verbose on test output
- do not run golangci-lint in system-tests
Use CDN-backed Debian mirror to make tests run faster hopefully for
everyone. Redirects might be important to know what exactly is going on
when items are being downloaded.