Provide Export .env option

### Description

Currently, it’s possible to import a .env file into an instance. Since environment variables are often set dynamically during flow development, it would be convenient to also have the ability to export the current environment variables of an instance to a .env file. This could also serve as a backup option.

### Which customers would this be available to

Team + Enterprise Tiers (EE)

### Have you provided an initial effort estimate for this issue?

I am not a FlowFuse team member