Fix/launchpad test dependency

[neolynx]

Description of the Change

System Tests (i.e. CreateMirror18Test) is depending on ppa.launchpad.net to be available, which is not always the case.

This change resolves that dependency by:

• making the ppa base urla configureable
• use a mirrored ppa on s3 for testing*

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.23%. Comparing base (a20eb68) to head (bba6bd7).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1568   +/-   ##
=======================================
  Coverage   77.22%   77.23%           
=======================================
  Files         161      161           
  Lines       15080    15083    +3     
=======================================
+ Hits        11646    11649    +3     
  Misses       2291     2291           
  Partials     1143     1143           

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