using advanced grammar to build towards a certain arch does not work with stage-snaps

[Oliver Grawert]

trying to build a snap using a foreign arch stage-snap like i.e. the pi gadget (so one does not need to cross compile the already existing binaries) seems to be missing the UBUNTU_STORE_ARCH variable set for the "snap download" call in snapcraft, so the snap is reported as not found...

while it is solvable through an override hack, it would be nice to be able to just do something like:

```
to: arm64
  stage-snaps:
    - pi
```

but instead something like below is required:
https://github.com/ogra1/custom-pi-gadget/blob/main/snap/snapcraft.yaml#L27