Support for env vars in `command:`

mvo

#1

It is currently not possible to use environment strings in the “command:” part of an app in snap.yaml. People are creating wrapper scripts for that right now. To make this easier I think we should allow it so that you can write e.g.:

name: some-snap
version: 1.0
apps:
 some-app:
  command: bin/some-app --data-dir $SNAP_DATA

The following PR implements that: https://github.com/snapcore/snapd/pull/3995


#2

Very reasonable, thanks for pushing it forward.