[ASP-4481] Fixed the installation of the agent's addons on its virtual environment

[fschuch]

An issue was identified when the addons included extra parameters for pip, for instance: "some-plugin==0.4.1 -i <value> --extra-index-url <value>". The logs on the charm show:

2024-08-14 20:53:20 DEBUG unit.jobbergate-agent/2.juju-log server.go:325 Emitting Juju event config_changed.
2024-08-14 20:53:20 DEBUG unit.jobbergate-agent/2.juju-log server.go:325 ## Installing jobbergate-addons: ['/srv/jobbergate-agent-venv/bin/pip', 'install', '-U', 'some-plugin==0.4.1 -i <value> --extra-index-url <value>']
2024-08-14 20:53:21 WARNING unit.jobbergate-agent/2.config-changed logger.go:60 ERROR: Invalid requirement: 'some-plugin==0.4.1 -i <value> --extra-index-url <value>'
2024-08-14 20:53:21 WARNING unit.jobbergate-agent/2.config-changed logger.go:60 Hint: It looks like a path. File 'some-plugin==0.4.1 -i <value> --extra-index-url <value>' does not exist.

After the changes on the PR we see:

2024-08-14 21:12:36 DEBUG unit.jobbergate-agent/2.juju-log server.go:325 Emitting Juju event config_changed.
2024-08-14 21:12:36 DEBUG unit.jobbergate-agent/2.juju-log server.go:325 ## Installing jobbergate-addons: ['/srv/jobbergate-agent-venv/bin/pip', 'install', '-U', 'some-plugin==0.4.1', '-i', '<value>', '--extra-index-url', '<value>']
2024-08-14 21:12:36 DEBUG unit.jobbergate-agent/2.config-changed logger.go:60 Looking in indexes: <value>, <value>
2024-08-14 21:12:36 DEBUG unit.jobbergate-agent/2.config-changed logger.go:60 Collecting some-plugin==0.4.1