default for snap download is global store (not brand store)

[Ian Johnson]

The default store ID for `snap install` is that of the model, which makes sense as it allows installation from the brand store for a device.

However the default store ID for `snap download` is the global store, which is inconsistent with `snap install` and seems wrong. I think that the _default_ store ID for all operations on a device with a model and a brand store should be the brand store's store ID.

For example to download a snap from the device's brand store, one would have to either have to know their brand store ID, or would have to do something like

```
UBUNTU_STORE_ID="$(snap known model | grep 'store: ' | awk '{print $2}')"
snap download brand-store-specific-snap
```

which is not obvious and a not so great user experience.

Note that this bug is simply about the default, obviously with the default brand store ID changed to the device's a user could still download any snap from the global store using

```
UBUNTU_STORE_ID="" snap download global-store-specific-snap
```

For a user on a device with a model and a brand store the more common use case would probably be to download snaps from their brand store, hence why it should be the default.

[Daniel Manrique (roadmr)]

Should this be reported against snapd instead?

https://bugs.launchpad.net/snapd/

The Snap Store honors the store requested by snapd via the model and serial assertion data (or forced via the UBUNTU_STORE_ID hack); if that's not being honored/sent for a snap download operation, there's not much the Snap Store can do, I think.

[Ian Johnson (anonymouse67)]

I was told that the decision to make snap download use the global store was a design decision by the store team, hence the bug here rather than snapd, but yes you are correct in that the offending code is actually in snapd.