chore: clean up backend testing CI

[gdalle]

• Add Pkg sources to backend test envs
• Switch draft backend testing to -O1
• Make draft PRs only run on 1.12

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.62%. Comparing base (5a3ea38) to head (adddf6e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1002      +/-   ##
==========================================
- Coverage   98.21%   90.62%   -7.59%     
==========================================
  Files         135      135              
  Lines        8000     7977      -23     
==========================================
- Hits         7857     7229     -628     
- Misses        143      748     +605     

Flag	Coverage Δ
DI	88.45% <ø> (-10.53%)	⬇️
DIT	96.22% <ø> (ø)

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