Without a valid MIGraphX cache dir, ONNX Runtime's MIGraphX EP reads an
uninitialized cache path and crashes on the first query rather than just
recompiling on every restart -- confirmed by hand while verifying the rocm
backend. Previously commented out as an opt-in perf tweak; it's actually
required for the rocm backend to work at all, so bakery-installed systems
need it set by default. No-op for cpu/npu/cuda backends.
Version bump: breadmill 0.2.1 -> 0.2.2.
e5922e9c90