Support container provisioner in toolbox

[thrix]

For Fedora Silverblue users it is common to run podman via flatpak-spawn --host which runs podman on the host system itself. This requires to pass the toolbox container name when running podman cp to correctly copy stuff from the toolbox container, where tmt is installed to the provisioned container.

Fixes #1020

Pull Request Checklist

• [x] implement the feature
• [ ] write the documentation
• [ ] extend the test coverage
• [ ] update the specification
• [ ] adjust plugin docstring
• [ ] modify the json schema
• [ ] mention the version
• [ ] include a release note