environment variabels export button

### Description

in the Flowfuse frontend under: instances => => Settings => environment
Currently you can import an .env file but can't export a .env file.

This reduces functionality.

Please add an export .env. button

### Which customers would this be available to

Everyone - CE/Starter/Team/Enterprise

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

I am not a FlowFuse team member