passthrough field being added to snap.yaml

[Ian M. Jones]

The last few automatic builds on snapcraft.io of my snippetpixie snap have been flagged for manual review each time because the resulting meta/snap.yaml has the `passthrough` field in its apps section.

Apparently this in the snapcraft.yaml...

```
apps:
  snippetpixie:
    command: bin/desktop-launch $SNAP/usr/bin/com.github.bytepixie.snippetpixie
    passthrough:
      autostart: snippetpixie_snippetpixie.desktop
    desktop: usr/share/applications/com.github.bytepixie.snippetpixie.desktop
    environment:
      GSETTINGS_SCHEMA_DIR: $SNAP/usr/share/glib-2.0/schemas
```

...results in the snap.yaml getting this...

```
apps:
  snippetpixie:
    autostart: snippetpixie_snippetpixie.desktop
    passthrough:
      autostart: snippetpixie_snippetpixie.desktop
```

This was discussed and the actual problem brought to my attention on the forum.

https://forum.snapcraft.io/t/blocked-from-updating-snap-due-to-some-auto-revision-i-didnt-trigger/15309/10

The snapcraft.yaml that has surfaced this bug can be found in the snippetpixie GitHub repo.

https://github.com/bytepixie/snippetpixie/blob/768d53503616a6cb5657bf296c3ce5ae2a06c348/snapcraft.yaml#L29

[Chris Patterson (cjp256)]

Fix PR: https://github.com/snapcore/snapcraft/pull/2914

[Chris Patterson (cjp256)]

Sorry for the troubles Ian! I have opened a PR that should address this issue and I expect we will backport it as well. Thank you for the report :)

[Sergio Schvezov (sergiusens)]

Part of 3.10