support: ensure that CHECKBOX_VENV_PATH is absolute
This patch fixes CHECKBOX_VENV_PATH to always be an absolute directory path. In the past there were cases where it could be set to a relative directory name and then setting up PROVIDERPATH and developing all of the providers would subsequently fail silently.
Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>