CI: test.yml: test-ibm: Use nproc instead of hard-coded number 2.

Author: junaruga

.github/workflows/test.yml

@@ -77,7 +77,7 @@ jobs:
           sudo apt-get update
           sudo apt-get install ruby-full bundler libyaml-dev
       - name: Install dependencies
-        run: sudo bundle install --jobs 2
+        run: sudo bundle install --jobs $(nproc)
       - name: Run test
         id: test
         run: rake