Make sure `/tmp` directory does not run out of scope before application ends

[adzialocha]

Moves TempDir instance into global scope to make sure it stays there for the whole applications lifetime.

Closes: #556

📋 Checklist

• [x] Add tests that cover your changes
• [x] Add this PR to the Unreleased section in CHANGELOG.md
• [x] Link this PR to any issues it closes
• [x] New files contain a SPDX license header

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (9bbcd08) 92.46% compared to head (f8d6145) 92.46%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## development #557 +/- ## ============================================ Coverage 92.46% 92.46% ============================================ Files 106 106 Lines 18039 18039 ============================================ Hits 16680 16680 Misses 1359 1359 ``` [see 2 files with indirect coverage changes](https://app.codecov.io/gh/p2panda/aquadoggo/pull/557/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=p2panda)

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