ci: Run podman command directly without wrapping it with prepare.sh

ci: Run podman command directly without wrapping it with prepare.sh

The prepare.sh script isn't currently used and forces us to make use
of sudo to switch the user inside the container from root to $USER
which created a problem on our Debian Slim-based containers which don't
have the 'sudo' package installed.
This patch removes the sudo invocation and instead runs the CMD
directly with podman.

Summary of the changes:
- move the corresponding env variables which we need to be set in the
  environment from the sudo invocation to the podman invocation
- pass --workdir to podman to retain the original behaviour we had with
  sudo spawning a login shell.
- MESON_OPTS env variable doesn't need to propagated to the execution
  environment anymore (like we had to do with sudo), because it's
  defined in the Dockerfile

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>