Fix/system tests

[neolynx]

Fixes the system tests permanently, by using a mirror on S3 for all the required files for the tests. No direct internet download, no more flaky tests.

Additionally:

• use ubuntu 22.04 runner in gitlab ci
• switch to gpg2
• add a system test docker image, and make targets

[codecov[bot]]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (f1649a6) 74.85% compared to head (a9da8d0) 74.84%. Report is 4 commits behind head on master.

Files	Patch %	Lines
pgp/openpgp.go	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1245 +/- ## ========================================== - Coverage 74.85% 74.84% -0.01% ========================================== Files 143 143 Lines 16187 16181 -6 ========================================== - Hits 12117 12111 -6 Misses 3134 3134 Partials 936 936 ```

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