apt: silence warnings when fetching apt-news

[lucasmoura]

Why is this needed?

When fetching the apt-news data, APT will try using the _apt user to store that information on the system. We are now creating a separate folder that is owned by _apt, which silences an APT warning when storing the apt-news data

Fixes: #3209
LP: #2070095

Test Steps

Check that the APT news related tests are still working and that the error message mentioned on #3209 no longer appears on Noble

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Bug References

• GitHub: Bug: apt_news download forced unsandboxed #3209
• Launchpad: #2070095

Confirm

• I've properly referenced all bugs that this PR fixes

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Test Updates

Unit Tests

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Documentation

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Does this PR require review from someone outside the core ubuntu-pro-client team?

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• No