Make the in-tree CLI use the in-tree PPP

Make the in-tree CLI use the in-tree PPP

We build an in-tree version of the CLI binary that refers to in-tree
versions of the plugins and other artefacts which currently doesn't
support the Pyhton plugin provider.

Add the logic to use the in-tree version of the Python plugin provider
using the LIBBABELTRACE2_PLUGIN_PROVIDER_DIR environment variable.

If the variable is already set, do not overwrite it. If the python
plugin is enabled, set it to the appropriate in-tree path, otherwise set
it to an invalid path so we don't load an hypothetical system installed
provider.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Change-Id: I4151c36fef90c791cc8b515233d0d3e8fefcdbd5
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1683
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>