cannot use build snaps that are marked private in the store

[Harald Sitter]

- have a build snap
- upload to store
- mark private
- try to use it as a build-snap in the snapcraft.yaml

snapcraft will claim the snap is not available even though it can be installed fine via snap install

```
Failed to install or refresh a snap: 'kde-frameworks-5-core18-sdk' does not exist or is not available on the desired channel 'latest/edge'. Use `snap info kde-frameworks-5-core18-sdk` to get a list of channels the snap is available on.
```

Being logged in with the same user in snapd and snapcraft makes no difference, it seems purely related to the snap being private.

[Alvaro Uria (aluria)]

This bug is also affecting BootStack's operations. We want to share a set of tools via snaps (previously shared via a private ppa), but copying the snap around is not maintainable.

[Onno Steenbergen (osteenbergen)]

Any update on this? In our case we use a separate store instead of a private snap. Same result, can't build snaps with "build-snaps" or "stage-snaps" requirements.