integration tests are using a hard-coded build-package version

[Leo Arias]

This is a followup from bug #1675447, which needed a quick immediate fix to not block the release.
So, this bug is for the proper fix.

We added a new integration test that checks that a deb with a specific version can be installed.
The problem here is that the archive will update and drop versions and will eventually fail.

The tests in test_build_package_version should query for the latest version of the package in the archive, and update the snapcraft.yaml with that value.

[Leo Arias (elopio)]

https://github.com/snapcore/snapcraft/pull/1269