ci/oldest: adapt to updated Debian release

There was a new Debian release few weeks ago, which actually forbids pip3 to install packages into a system location (be it a system or a user's global dir). Follow their advice and use venv to install meson locally.

Note: an alternative would be to pass --break-system-packages to pip3.

Signed-off-by: Dmitry Baryshkov dmitry.baryshkov@linaro.org