[llvm] [llvm][test][NFCI] Use absolute build path instead of a relative path (PR #159126)

[Raul Tambre via llvm-commits]

tambry wrote:

@mgorny That's what I get for not waiting my local build and test run to finish... Finally fixed: #159923. :crossed_fingers: 

https://github.com/llvm/llvm-project/pull/159126