Use ${LLVM_COV} variable instead of hardcoded llvm-cov

[Copilot]

Addresses feedback from #385 to use the CMake variable found by Findverification-coverage.cmake instead of hardcoding the tool name.

Changes

• Pass LLVM_COV to generated coverage script via set(LLVM_COV "${LLVM_COV}")
• Replace hardcoded llvm-cov with \${LLVM_COV} in all execute_process() commands (4 occurrences)

This matches the existing pattern for LLVM_PROFDATA and ensures the build uses the tool path discovered by CMake's find_program() rather than assuming it's in PATH.

# Before
execute_process(
    COMMAND llvm-cov show -format=html ...
)

# After  
execute_process(
    COMMAND \${LLVM_COV} show -format=html ...
)

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