build.zig: fix test-stage2 regression with -Denable-llvm

Regressed in cc9eb9e90fec961a74e90e1ab3d66a7b8ec14f47 due to a merge
conflict.
23567cdd98Andrew Kelley committed on 2/18/2022, 3:48:09 AMparent0bb2260
1 file changedLine totals unavailable