Build HPL serially to avoid upstream make races

iso/builder/build-hpl.sh

@@ -367,7 +367,7 @@ gcc -O2 -c -o "${BUILD_TMP}/mpi_stub.o" "${BUILD_TMP}/mpi_stub.c"
 
 # build HPL
 echo "=== building HPL ${HPL_VERSION} ==="
-make -j"$(nproc)" arch=bee
+make arch=bee
 
 XHPL_BIN="bin/bee/xhpl"
 [ -x "${XHPL_BIN}" ] || { echo "ERROR: xhpl not found after build"; exit 1; }