Keep precompile PCH generation-local

scripts/compile_wasm_unit

@@ -9,6 +9,7 @@ DEST_DIR="$2"
 SRC_FN="$3"
 PP_FN="$4"
 WASM_FN="$5"
+UNIT_CACHE_DIR="${6:-$DEST_DIR}"
 
 SDK=${WASI_SDK:-/opt/wasi-sdk}
 ABI_VERSION=${UCE_UNIT_ABI_VERSION:-$(awk '/^#define UCE_WASM_CORE_ABI_VERSION / {print $3; exit}' src/wasm/abi.h)}
@@ -19,7 +20,7 @@ MODULE_TMP="$DEST_DIR/$PP_FN.uce-module.txt"
 WASM_TMP="$DEST_DIR/$WASM_FN.tmp.$$"
 MAP_TMP="$DEST_DIR/$WASM_FN.source-map.tmp.$$"
 PCH_ENABLED=${UCE_WASM_UNIT_PCH:-1}
-PCH_DIR=${UCE_WASM_PCH_DIR:-/tmp/uce/wasm-w2/pch}
+PCH_DIR=${UCE_WASM_PCH_DIR:-$UNIT_CACHE_DIR/.pch}
 COMMON_FLAGS=(
 	--target=wasm32-wasip1
 	-fPIC -fvisibility=hidden -fvisibility-inlines-hidden