[App] Use environment variables from args when loading apps for the CloudRuntime

## What does this PR do?

Passes environment variables to the `CloudRuntime` when loading an app from a file. Allows apps / works / flows to be properly configured when passing variables at runtime.

### Limitations

- Only implemented for the `CloudRuntime`

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## PR review

Anyone in the community is welcome to review the PR.
Before you start reviewing, make sure you have read the [review guidelines](https://github.com/Lightning-AI/lightning/wiki/Review-guidelines). In short, see the following bullet-list:

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cc @borda