Automatic env var detection from flows

### Epic

_No response_

### Description

**As a:** User

**I want:** the environment variable list in project settings to be pre-populated with the names of env vars the current flows reference.

**So that:** I don't have to remember what they all are

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Currently we have a very disjointed UX around environment variables. They provide the main way for customising a flow that is deployed to multiple places. But we currently rely on the user just knowing what env vars need to be set to do anything.

As an improvement on this, the Env Var settings page could be pre-populated with a list of env vars the flows reference.

There is a related story in this are for Devices.

### Which customers would this be availble to

All Users, (CE)

### Acceptance Criteria

_No response_