Make ORT_MIGRAPHX_MODEL_CACHE_PATH unconditional in packaged systemd unit

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.

Cargo.lock

@@ -133,7 +133,7 @@ dependencies = [
 
 [[package]]
 name = "breadmill"
-version = "0.2.1"
+version = "0.2.2"
 dependencies = [
  "breadsearch-shared",
  "hex",

breadmill/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "breadmill"
-version = "0.2.1"
+version = "0.2.2"
 edition = "2021"
 license = "MIT"
 

packaging/breadmill.service

@@ -5,10 +5,11 @@ After=default.target
 
 [Service]
 Type=simple
-# Uncomment if built with --features rocm: persists MIGraphX's compiled-kernel
-# cache across restarts (each new sequence length otherwise costs a ~60-120s
-# recompile). See README.md#gpu-backend-notes.
-#Environment=ORT_MIGRAPHX_MODEL_CACHE_PATH=%h/.cache/breadsearch/migraphx-cache
+# Required (not just a perf tweak) when backend = "rocm": without a valid
+# cache dir, ONNX Runtime's MIGraphX EP reads an uninitialized cache path and
+# crashes on the first query instead of just recompiling every restart. Inert
+# for cpu/npu/cuda backends. See README.md#gpu-backend-notes.
+Environment=ORT_MIGRAPHX_MODEL_CACHE_PATH=%h/.cache/breadsearch/migraphx-cache
 ExecStart=%h/.cargo/bin/breadmill
 Restart=on-failure
 RestartSec=5