[FEATURE] Event sources' process.env should contain system environment variables

Currently `process.env` in event sources isn't populated with [environment variables](https://docs.pipedream.com/environment-variables/). I'd like to have those env vars populated in event sources so I can use them.

Start by reading the linked environment variables documentation and locating the event-source runtime entry point where process.env is exposed; the issue names no files or tests. Verify the change with an event source that reads a documented environment variable and confirm that system environment variables are available to it.

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