GHA: Show b2 command invocation

[Flamefire]

To aid debugging CI failures use set -x to show the actual build command issued.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.76%. Comparing base (8fbdb8a) to head (f2a4c36).
Report is 7 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #245   +/-   ##
========================================
  Coverage    94.76%   94.76%           
========================================
  Files           69       69           
  Lines         9031     9031           
========================================
  Hits          8558     8558           
  Misses         473      473           

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