Install fails in a poetry project when `virtualenvs.create` is `false`

If the user configures poetry with virtualenvs.create = false (e.g. in ~/.config/pypoetry/config.toml), poetry writes a warning to stdout when appmap-agent-init gets run:

root@d6e62d18d48a:/Users/ajp/src/cisco-open/otel-py# poetry run appmap-agent-init
Skipping virtualenv creation, as specified in config file.
{"configuration": {"filename": "appmap.yml", "contents": "name: otel-py\npackages:\n- path: tests\n- path: cisco_telescope\n"}}

This causes installation to fail because we try to parse the contents of stdout JSON.

Start at the appmap-agent-init entry point and reproduce the command in a Poetry project with virtualenvs.create=false. Trace how stdout is captured and parsed; done means Poetry's warning no longer causes the configuration JSON parsing to fail.