add nightly edge releases

[efiop]

We used to push to edge on every commit in master, but there is really no good reason to do that. Nightly edge releases will be enough. But there is also the question if anyone is even using these builds, so I'm leaving this issue open to collect requests/feedback/demand.

If we do decide to implement these, removing source-tag from the snap/snapcraft.yaml for the build should do the trick.