chore: add rekor prod TUF system test

[kipz]

• Now the rekor public key is in our TUF roots (in testing channel), we can test that the fallback from the in-memory key to the prod key works. This is run on CI only
• Also validates that the testing prefix works in prod

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.95%. Comparing base (02b8063) to head (6c16d01).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #176      +/-   ##
==========================================
+ Coverage   69.71%   69.95%   +0.23%     
==========================================
  Files          44       44              
  Lines        2533     2533              
==========================================
+ Hits         1766     1772       +6     
+ Misses        476      469       -7     
- Partials      291      292       +1     

Flag	Coverage Δ
unittests	69.95% <ø> (+0.23%)	⬆️

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