Inline kernel build into test-coverage job

[d-e-s-o]

With recent changes, the build-linux action outputs the build directory path directly instead of uploading an artifact. Since it is a composite action, its steps run in the caller's job and the build directory is already on disk.
Inline the build-linux step into the test-coverage job, removing the separate build-linux-kernel job along with the artifact download via curl. Doing so simplifies things a bunch and eliminates the scheduling issues we have seen where build-linux-kernel, despite being on the critical path, was sometimes scheduled to run only after a bunch of other jobs, which artificially worsened total job runtime.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.51%. Comparing base (ebba953) to head (8cd4d47).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1543      +/-   ##
==========================================
+ Coverage   95.50%   95.51%   +0.01%     
==========================================
  Files          56       56              
  Lines        9534     9534              
==========================================
+ Hits         9105     9106       +1     
+ Misses        429      428       -1     

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