UI: Bugs in generating agent deploy script

[aarthy-dk]

Some bugs noted on the Integrations panel and generated script:

• For legacy agents, the EVENTS_API_HOST variable is disabled.
• For Amazon S3, the variable is generated as AWS_SECRET_KEY, but the agent expects SECRET_KEY.
• For Linux systems, agents are unable to connect to the Observability app running on local host. This is because host.docker.internal:host-gateway needs to be added as in extra_hosts on the docker compose file.