Expand coverage to sccache-dist tests

[samuel-jimenez]

Add testing matrix option for dist-tests.
Configures LLVM_PROFILE_FILE environment variable to collect coverage results.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.43%. Comparing base (0cc0c62) to head (fb27bc8).
Report is 155 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2350       +/-   ##
===========================================
+ Coverage   30.91%   67.43%   +36.52%     
===========================================
  Files          53       65       +12     
  Lines       20112    35396    +15284     
  Branches     9755        0     -9755     
===========================================
+ Hits         6217    23869    +17652     
- Misses       7922    11527     +3605     
+ Partials     5973        0     -5973     

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[sylvestre]

thanks!