tests: add scenario to assure custom APT configuration is respected

[renanrodrigo]

Why is this needed?

This PR solves all of our problems because it ensures custom APT configuration is respected by the pro client when done globally, whether through proper system configuration or using environment variables.

Test Steps

CI should be green, as the new integration test runs on X/N lxd-container

Checklist

• I have updated or added any unit tests accordingly
• I have updated or added any integration tests accordingly - that is the whole point
• Changes here need to be documented, and this was done in: docs: add explanation on how to use custom APT configuration #3151

Does this PR require extra reviews?

• Yes
• No

[github-actions]

Jira: This PR is not related to a Jira item. (The PR title does not include a SC-#### reference)

GitHub Issues: No GitHub issues are fixed by this PR. (No commits have Fixes: #### references)

Launchpad Bugs: No Launchpad bugs are fixed by this PR. (No commits have LP: #### references)

Documentation: Documentation changes addressed in #3151

👍 this comment to confirm that this is correct.

[renanrodrigo]

I chose Xenial and Noble because

• Xenial older, Noble newest
• Xenial to Jammy will have APT behaving exactly the same, no need to run them all IMHO
• Noble carries the new deb822 config files, and we make sure it still works even there

[orndorffgrant]

LGTM