`stage-snaps: mir-kiosk-x11` segfaults on `base: core18`

I did a "s/adopt-info: mir-kiosk/version: test/g" change and built the snap successfully with "snapcraft --use-lxd"

sergiusens@imladris:~/repositorios/snaps/mir-kiosk$ snapcraft --use-lxd
Launching a container.
Waiting for container to be ready
Waiting for network to be ready...
snapd is not logged in, snap install commands will use sudo
snap "core18" has no updates available
Downloading snap 'mir-kiosk-x11'
Pulling mir-kiosk
+ snapcraftctl pull
Building mir-kiosk
+ snapcraftctl build
Staging mir-kiosk
+ snapcraftctl stage
Priming mir-kiosk
+ snapcraftctl prime
'grade' property not specified: defaulting to 'stable'.
Snapping |
Snapped try-kiosk-x11_test_amd64.snap

Same results when using multipass. Any more details to add the how to reproduce?

Hmm I wonder if it kept the older revision around?

Will try again tomorrow and report back.

OK so trying this:

```
name: try-kiosk-x11
version: none
summary: Segfault on stage-snaps
description: |
  This shows snapcraft segfaulting on stage-snaps
confinement: strict
base: core18

parts:
  mir-kiosk:
    plugin: nil
    stage-snaps:
      - mir-kiosk-x11
```

Fails as expected, because mir-kiosk-x11 is core20-based. But changing it to `/latest/beta` and rebuilding without cleaning will segfault again. After clean, it works correctly.

We should check the stage-snap change and cleanup and warn when the build-base used does not match that of the stage-snap

I can't reproduce the segfault with current snapcraft and packages in any if the described scenarios. Is this still a concern? If so, could you provide new information on how to reproduce?

@cmatsuoka you'd need a snap with one channel based `core18`, another `core20`, and switch between them without cleaning.

We don't currently have such a situation, so don't have an easy reproducer today.

Thanks, I'll prepare a couple of test snaps to reproduce this scenario.