Use getconf [skip ci]

Author: ankane

.github/workflows/build.yml

@@ -32,5 +32,5 @@ jobs:
           cd ~
           wget -q -O libtorch.zip https://download.pytorch.org/libtorch/cpu/libtorch-cxx11-abi-shared-with-deps-$LIBTORCH_VERSION%2Bcpu.zip
           unzip -q libtorch.zip
-      - run: MAKE="make -j$(nproc)" bundle exec rake compile -- --with-torch-dir=$HOME/libtorch
+      - run: MAKE="make -j$(getconf _NPROCESSORS_ONLN)" bundle exec rake compile -- --with-torch-dir=$HOME/libtorch
       - run: bundle exec rake test