snapcraft cleanbuild produces incorrect architecture snap

[Ian Johnson]

When using the stable release of the snapcraft snap, if one uses the advanced architectures yaml like:

```
architectures:
  - build-on: amd64
    run-on: amd64
  - build-on: armhf
    run-on: armhf
```

and without a base specified so snapcraft operates in legacy mode, then snapcraft cleanbuild will produce a "multi" file snap name like "snap-name_0.1_multi.snap", even though the snap.yaml in the snap itself has the correct architecture specification.

This can be reproduced on amd64 with snapcraft stable snap (revision 2496), and a basic snap without `base` and with the above architectures snippet and specificaly using snapcraft cleanbuild

[Sergio Schvezov (sergiusens)]

cleanbuild is no longer supported, all builds by default now transparently "cleanbuild"