The test suite spawns the grep binary inside per-test temporary
directories (via uutests). With a relative LLVM_PROFILE_FILE, each
subprocess wrote its .profraw into that tempdir, which gets deleted
when the test ends, so the binary executing the src/ code left no
coverage data and grcov reported every line as a miss (0%).
Anchoring the path to the workspace root makes the profraw files
survive, raising reported coverage from 0% to ~96%.