ci: use an absolute LLVM_PROFILE_FILE path so coverage includes the sed binary

[DePasqualeOrg]

The integration tests run the sed binary through uutests, which starts each command in a temporary directory that is deleted when the test ends. Because LLVM_PROFILE_FILE is a relative path, every run of the binary writes its .profraw file into that directory, and its coverage is lost. Only the unit tests, which run in the workspace, are counted.

As a result, Codecov reports src/sed/processor.rs at about 28%. Code that many tests run, such as the q, Q and = commands and the automatic printing of the pattern space, is shown as never executed, and codecov/patch fails on most changes to command behavior.

This PR anchors the path to the workspace, so the files survive the test and grcov finds them. uutils/grep#5 made the same change, which raised its reported coverage from 0% to about 96%.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.82%. Comparing base (1113479) to head (3f802af).

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #591       +/-   ##
===========================================
+ Coverage   83.86%   94.82%   +10.95%     
===========================================
  Files          14       14               
  Lines        7203     7203               
  Branches      424      424               
===========================================
+ Hits         6041     6830      +789     
+ Misses       1157      372      -785     
+ Partials        5        1        -4     

Flag	Coverage Δ
macos_latest	96.36% <ø> (+11.38%)	⬆️
ubuntu_latest	96.30% <ø> (+11.12%)	⬆️
windows_latest	0.00% <ø> (ø)

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