Use getconf [skip ci]

Author: ankane

.github/workflows/build.yml

@@ -23,5 +23,5 @@ jobs:
         with:
           ruby-version: ${{ matrix.ruby }}
           bundler-cache: true
-      - run: MAKE="make -j$(nproc)" bundle exec rake compile
+      - run: MAKE="make -j$(getconf _NPROCESSORS_ONLN)" bundle exec rake compile
       - run: bundle exec rake test